View Full Version : Pistons and violent early extraction...
Reagans Rascals
03-12-12, 21:26
So I have a quick question regarding piston operated uppers.
In a normal D.I. upper, as gas is imparted into the gas tube, it travels down through the gas key, into the cavity in the carrier, and pushes the bolt itself forward. This eases extraction while also thrusting the BCG rearward. So essentially the gas is used to force the carrier rearward, bolt forward, rotate and unlock the bolt from the breach, then extract the round as it travels rearward.
However, in a piston system, the gas key is replaced with just a flat piston key which the short stroke rod impacts and forces the BCG rearward... so there is no forward motion of the bolt to help unlock and ease the stress on the lugs right? It just rips it right out... which to me sounds very detrimental to the system... it extracts the round before it may be at the optimal size to extract ( forward motion of the bolt by a D.I. system provides sufficient time for the case to cool and shrink) and it just shoves the carrier to the rear and basically just rips the lugs out, causing them to sheer against the extension lugs until they eventually unlock and allow the bolt to leave.
Basically piston systems just use brute force to rip the bolt/shell out of the chamber... seems to me like carrier tilt would be a drop in the bucket compared to the stresses that type of action has on the rifle, so the various retro-fit kits available would be about as beneficial as using a filled diaper to lube your gun...
Is this assessment correct?
The gas rings on the bolt are replaced by a spring, constantly pushing the bolt forward. What you've stated may very well be the reason behind it.
Someone else may know better/more...
militarymoron
03-12-12, 21:46
some piston systems like the PWS have a spring behind the bolt that push it forward lightly, ensuring that it's in its most forward position in the barrel extension until the rearward movement of the carrier unlocks it.
http://primaryweapons.com/store/pc/viewPrd.asp?idproduct=132&idcategory=19
in my writeup (http://www.militarymorons.com/weapons/ar.uppers3.html#atac2) back in feb 2010, this is what i wrote:
"The new PWS 5.56 AR Piston Bolt is heat treated to mil spec and MP tested. The bolt has no gas ring groove, but instead has a modified shoulder that is designed to accept a spring behind the bolt. The spring provides a slight amount of pressure that ensures that the bolt is in its most forward position in the receiver extension when in battery. One of the reasons why I left the gas rings on the bolts of my piston guns was to provide some drag inside the carrier, to help reduce the cam pin slamming into the carrier. During the cycle, a bolt without gas rings is free to 'bounce' back and forth a minute amount, and the cam pin will slam around in its channel and at the end of its movement.
When in battery, there is still a minute bit of slop in the bolt, and at the time of firing, it may not be in its most forward position. On a DI system, the gas expands into the space between the tail of the bolt and the bolt carrier, and pushes the bolt and carrier apart, ensuring that the bolt is pushed forward into the receiver extension. On a piston gun, the bolt is just unlocked by the backward motion of the carrier when the piston/op rod moves back. If the bolt isn't in its most forward position, the cam pin can make premature contact with the cam pin recess on the inside of the receiver, causing more wear than with a DI gun. The wear might be self-limiting and not an issue, but it also implies more stress and wear on the components than if it was prevented or reduced. The PWS piston bolt with spring is designed to reduce this by keeping spring pressure on the bolt so the slop is taken up. The bolt is for use in piston rifles only, and will not work in DI rifles."
Robb Jensen
03-12-12, 21:47
Piston ARs are like the rotary engine of the car world but of the rifle world. In theory they're better....but in execution not so much. There is no "free lunch". If you want a piston rifle buy one that was meant to be one (AK, FAL, M1a, SCAR).
Reagans Rascals
03-12-12, 21:52
some piston systems like the PWS have a spring behind the bolt that push it forward lightly, ensuring that it's in its most forward position in the barrel extension until the rearward movement of the carrier unlocks it.
http://primaryweapons.com/store/pc/viewPrd.asp?idproduct=132&idcategory=19
in my writeup (http://www.militarymorons.com/weapons/ar.uppers3.html#atac2) back in feb 2010, this is what i wrote:
"The new PWS 5.56 AR Piston Bolt is heat treated to mil spec and MP tested. The bolt has no gas ring groove, but instead has a modified shoulder that is designed to accept a spring behind the bolt. The spring provides a slight amount of pressure that ensures that the bolt is in its most forward position in the receiver extension when in battery. One of the reasons why I left the gas rings on the bolts of my piston guns was to provide some drag inside the carrier, to help reduce the cam pin slamming into the carrier. During the cycle, a bolt without gas rings is free to 'bounce' back and forth a minute amount, and the cam pin will slam around in its channel and at the end of its movement.
When in battery, there is still a minute bit of slop in the bolt, and at the time of firing, it may not be in its most forward position. On a DI system, the gas expands into the space between the tail of the bolt and the bolt carrier, and pushes the bolt and carrier apart, ensuring that the bolt is pushed forward into the receiver extension. On a piston gun, the bolt is just unlocked by the backward motion of the carrier when the piston/op rod moves back. If the bolt isn't in its most forward position, the cam pin can make premature contact with the cam pin recess on the inside of the receiver, causing more wear than with a DI gun. The wear might be self-limiting and not an issue, but it also implies more stress and wear on the components than if it was prevented or reduced. The PWS piston bolt with spring is designed to reduce this by keeping spring pressure on the bolt so the slop is taken up. The bolt is for use in piston rifles only, and will not work in DI rifles."
great write up, I've never heard of the replacement of gas rings with a spring before, but it makes sense
I guess the HK416 uses a spring encompassing the firing pin to keep the bolt in the forward position
http://i1058.photobucket.com/albums/t403/jwmassaro/HKBCG_08.jpg
"I guess the HK416 uses a spring encompassing the firing pin to keep the bolt in the forward position"
umm... no
The spring keeps the firing pin away from the carrier, and does absolutely nothing with regards to "pushing the bolt forward."
ICANHITHIMMAN
03-12-12, 22:31
I have the PWS spring on my LMT bolt I have not had and issue with it.
Reagans Rascals
03-12-12, 22:40
"I guess the HK416 uses a spring encompassing the firing pin to keep the bolt in the forward position"
umm... no
The spring keeps the firing pin away from the carrier, and does absolutely nothing with regards to "pushing the bolt forward."
how do they go about keeping the bolt forward during extraction? Or do they not worry about it and simply just use brute force of the piston pushing the BCG rearward?
Piston ARs are like the rotary engine of the car world but of the rifle world. In theory they're better....but in execution not so much. There is no "free lunch". If you want a piston rifle buy one that was meant to be one (AK, FAL, M1a, SCAR).
Quoted for absolute truth.
"I guess the HK416 uses a spring encompassing the firing pin to keep the bolt in the forward position"
umm... no
The spring keeps the firing pin away from the carrier, and does absolutely nothing with regards to "pushing the bolt forward."
Well, I would not be so sure.
Although the purpose of the firing pin spring is to push the firing pin back in order to activate the firing pin safety after firing, the firing spring also spring loads the bolt. If it pushes it forward, I can't really say, since I can't see inside the gun....
Anyways, there is no sign of excess stress on my BCG, barrel extension or inside the upper on my gun. No signs of carrier tilt either. I have probably close to 10k through it....so I guess the design works, huh?
An Undocumented Worker
03-13-12, 00:14
I doubt you could use any spring in a piston AR bolt carrier that would provide as much thrust on the bolt without pushing the carrier out of battery as a DI AR does with gas pressure. We are basically talking thousands of PSI vs a spring that could only supply at most 12 pounds of force before it starts to overcome the action spring.
As to whether the gas pressure behind the bolt actually pushes it off of the barrel extension lugs, I really couldn't say, but I'm sure it at least cuts down the load on the lugs during unlocking.
I doubt you could use any spring in a piston AR bolt carrier that would provide as much thrust on the bolt without pushing the carrier out of battery as a DI AR does with gas pressure. We are basically talking thousands of PSI vs a spring that could only supply at most 12 pounds of force before it starts to overcome the action spring.
True.
Anyways, after seeing and inspecting a "few" 416's, I can say that the issue the OP has is really a non-issue, with that system at least.
CumbiaDude
03-13-12, 00:49
If you want a piston rifle buy one that was meant to be one (AK, FAL, M1a, SCAR).Huge +1 to this. I love the people who take an AR and then make it piston driven and get .300 AAC... isn't that just the AKM? 125 gr bullet, 2300 fps, piston gun....
Keep your ARs ARs - Direct impingement and 5.56. That's my opinion, anyway... not that I know anything :)
While the gases do push against the bolt, they cannot push the bolt forward! Although the bullet has exited the barrel by the time the expansion chamber fills with enough pressure to start the carrier moving, the pressure in the bore is still greater. Also, the spent case has expanded enough to take up what little slack there was and resist any forward motion of the bolt.
It's also a fallacy to believe the AR does not have a piston. It's piston is part of the bolt and the carrier is a cylinder. The piston is inline with the bolt and bore, reducing muzzle climb induced by off center recoil forces and reciprocating mass.
The AR does not feature primary extraction, a mechanical camming action that starts extraction as the bolt begins unlocking. Many other designs do, such as the M14 and most bolt actions. Whether the AR uses the original inline piston or an aftermarket offset piston relocated to the gas block, it must rely on "brute force" to yank the spent cartridge from the chamber
I have one of each. I like my DI. I wanted a piston gun(just because). I like AR's not ak, fal etc. etc. blah blah. So....I built a piston AR!;) and I shoot the shit out of em both and don't worry about how many milliseconds longer the damn piston bolt lugs are taking to unlock from the exstension lugs than the DI bolt is or how forceful one process is over the other!;)
Robb Jensen
03-13-12, 04:24
I have one of each. I like my DI. I wanted a piston gun(just because). I like AR's not ak, fal etc. etc. blah blah. So....I built a piston AR!;) and I shoot the shit out of em both and don't worry about how many milliseconds longer the damn piston bolt lugs are taking to unlock from the exstension lugs than the DI bolt is or how forceful one process is over the other!;)
Shooting the gun takes a room temperature I.Q., understanding it is something you have to do on your own. You're starting to veer out of your lane.
justin_247
03-13-12, 05:37
I have one of each. I like my DI. I wanted a piston gun(just because). I like AR's not ak, fal etc. etc. blah blah. So....I built a piston AR!;) and I shoot the shit out of em both and don't worry about how many milliseconds longer the damn piston bolt lugs are taking to unlock from the exstension lugs than the DI bolt is or how forceful one process is over the other!;)
You should care, because those milliseconds equal material stresses that the weapon was not originally designed for.
Shooting the gun takes a room temperature I.Q., understanding it is something you have to do on your own. You're starting to veer out of your lane.
that is a good one. :laugh:
You should care, because those milliseconds equal material stresses that the weapon was not originally designed for.
And how exactly did you come to that conclusion after reading MistWolf's post? :confused:
How is the unlocking process of the bolt in the M16 FOW different than in an op-rod design, considering that the bolt cannot move forward?
Also, I just spoke to the main armorer here, and he reiterated that there is no difference in the unlocking process between the two systems; the gas pushes on the carrier and the op-rod pushes on the carrier. The cam pin follows the cam track as the carrier moves to the rear, and this causes the cam pin and bolt to rotate until the lugs on the bolt are no longer in line with the locking lugs in the barrel extension.
There will be wear on the locking lugs on both systems.
Why was this thread started again? Smells alot like the same old DI vs. piston crap again.
Shooting the gun takes a room temperature I.Q., understanding it is something you have to do on your own. You're starting to veer out of your lane.
Yes, I'm sure Stoner created the "GI proof" rifle in the hopes that soldiers would be kicking back, contemplating the nuances of the mechanics of its operation for decades to come.
constructor
03-13-12, 10:31
One thing to check- Shoot your AR in the dark, if flames shoot out of the ejection port the spent case is being pulled out of the chamber while the bullet is still in the barrel and high pressure gases are exiting the chamber meaning the pressure is still high in the chamber, the case has not contracted from the chamber walls and at some point the extractor will fail due to excess stress from ripping the case out of the chamber. It doesn't matter if the rifle is a DI or piston either can be over-gassed. If the carrier is moving while the bullet is in the bore it can effect accuracy.
Robb Jensen
03-13-12, 10:51
Yes, I'm sure Stoner created the "GI proof" rifle in the hopes that soldiers would be kicking back, contemplating the nuances of the mechanics of its operation for decades to come.
No he designed the gun and it's issued to GIs. Most people issued a gun have very little understanding of it's operation and why it does what it does.
If you're here to debate and discuss the design by all means do. If you're here to be an ass your stay will be very short...last warning.
Yes, I'm sure Stoner created the "GI proof" rifle in the hopes that soldiers would be kicking back, contemplating the nuances of the mechanics of its operation for decades to come.
IMHO, stoner did NOT create a "GI PROOF" weapon. The AR15/M16 is much like the 1911. It is a great gun, but is not for everyone. A high IQ is needed in order to make this gun(s) run well.
C4
Reagans Rascals
03-13-12, 11:55
Why was this thread started again? Smells alot like the same old DI vs. piston crap again.
My entire reason for starting this thread was to ask the question, how do dedicated piston uppers deal with unlocking the bolt and extraction.
To my knowledge they just rip the bolt/shell out by brute force, where as a D.I. gun uses gas pressure to ease the unlocking of the bolt and provide dwell time to allow the case to cool for proper extraction.
What do piston systems like the HK416 employ to facilitate unlocking and extraction.
Has nothing to do with D.I. vs. Piston... this is a technical question
Both systems operate the same way, ie something is exerting force on the carrier. In a DI it's the gas, in the op-rod design it's the op-rod. Unlocking happens when the bolt carrier starts moving to the rear, and the cam pin follows the cam track in the carrier, thus rotating the bolt so that the lugs on the bolt does not align with the lugs in the barrel extention.
There is no forward movement of the bolt in either system.
Both systems incur wear on bolt lugs.
In my 4 years with the HK416 in service, I have not seen a single broken bolt lug.....
There were some issues with the initial 416's, with reports of bolt lug breakage after only 5000 rounds. That happened because HK used a dfferent metal and hardening process on the bolt. They switched back to the original process and metal, and now the bolts last a lot longer.
sinlessorrow
03-13-12, 12:43
IMHO, stoner did NOT create a "GI PROOF" weapon. The AR15/M16 is much like the 1911. It is a great gun, but is not for everyone. A high IQ is needed in order to make this gun(s) run well.
C4
Well said Grant.
The M16Fow is certainly not GI proof. The system takes some knowledge to keep it running, thats why people who know the system will never have issues and the ones that dont will.
Some people think dry lubes are best for the M16 but if you know the system you will know it needs a good lube to run well.
Like Grant said, the M16 like the 1911 requires knowledge of the system to keep it running, and if you have that knowledge it will never give you troubles.
...One thing to check- Shoot your AR in the dark, if flames shoot out of the ejection port the spent case is being pulled out of the chamber while the bullet is still in the barrel...
There is no way the expansion chamber will pressurize enough to start moving the carrier before the bullet can travel from gas port to muzzle. The distance from gas port to muzzle is shorter than it is from gas port to carrier. Also, if I remember correctly, the velocity of the supersonic gases drop to subsonic after passing through the gas port. In the race between bullet to muzzle and gas to expansion chamber, the bullet wins every time.
When you see flames coming out of the chamber, it's for the same reason you see flames coming out of the muzzle. It's the spent gases igniting as they mix with the oxygen in the atmosphere
...To my knowledge they just rip the bolt/shell out by brute force, where as a D.I. gun uses gas pressure to ease the unlocking of the bolt and provide dwell time to allow the case to cool for proper extraction...
The so called "piston" uppers actually work the same way as the Stoner system. Both use a piston and cylinder to turn the energy of the expanding gases into mechanical energy. The difference between the two systems is that the Stoner system eliminates the actuator rod and places the piston inline with the bolt & bore. The Stoner system may have a "softer" extraction as it's expansion chamber (cylinder) has a larger volume. In either case, the bullet has long left the bore before the reciprocating mass has a chance to start moving
...Unlocking happens when the bolt carrier starts moving to the rear...
The bolt does not begin unlocking as soon as the carrier begins moving. The groove in the carrier which engages the bolt cam is straight for a short distance, then angles which delays the unlocking of the bolt
Arctic, your a bit off.
Don't blame your armorer.. they are constrained by the limit of education & most are lego assembling monkeys.
Look at where the piston is in the standard AR. In the middle of the bolt carrier. As gas goes through the key, into the chamber (within the carrier) it pushes in all directions.
- The bolt will be pushed forward and the carrier rearward... gas venting out the side holes in the carrier as the gas rings pass.
- as the carrier is pushed rearward the cam path unlocks the bolt, aligns it with openings in barrel extension
- momentum is keeping the carrier moving rearward.
The difference was very clear with early hk416's which dug chunks out of the inlet, for the cam pin, in the upper receiver.
BTW the 416 can more correctly be referred to as a gas tappet, not oprod. Ala the G36 some of your countrymen use
- The bolt will be pushed forward and the carrier rearward...
There will be forces pushing forward against the piston/bolt, but not great enough to cause the bolt to move within the barrel extension.
1- The resistance to pushing the bolt forward is much greater than the resistance to moving the carrier rearward.
2- A round in the chamber usually fits snugly enough to take up any slack there may be between the bolt and barrel extension, even before firing.
3- The expanded case will keep the pressure in the expansion chamber from moving the bolt forward. During firing, the case is pushed rearward against the bolt face and expands to stick to the chamber walls. The case should have shrunk by the time the gases enter the expansion chamber, but the case has expanded to fill the chamber. The only way the bolt can be moved forward is to crush the brass.
4- While the pressure in the entire system is dropping because the bullet has uncorked it, pressure in the carrier is still less than the pressure in the chamber and bore.
The gases in the expansion chamber cannot move the bolt forward in the barrel extension
The bolt does not begin unlocking as soon as the carrier begins moving. The groove in the carrier which engages the bolt cam is straight for a short distance, then angles which delays the unlocking of the bolt
Yeah, I know, poor wording on my part perhaps. I didn't mean that unlocking starts when the carrier moves to the rear, I was trying to say that it was a result of the cam pin following the cam track, and the resulting rotation of both cam pin and bolt. If that makes sense.....I suck at english firearms nomenclature....sorry....
@dano5326:
We still have cam pin recess wear marks, if that is what you are talking about in regards to inlet:
http://img195.imageshack.us/img195/2415/p3130002j.jpg
Don't blame your armorer.. they are constrained by the limit of education & most are lego assembling monkeys.
Well, this guy is actually quite experienced, having been in the business for over 30 years, with several armorer courses from various manufacturers (FN, HK, Colt Canada/Diemaco, Glock). He does know his stuff.
Look at where the piston is in the standard AR. In the middle of the bolt carrier. As gas goes through the key, into the chamber (within the carrier) it pushes in all directions.
- The bolt will be pushed forward and the carrier rearward... gas venting out the side holes in the carrier as the gas rings pass.
- as the carrier is pushed rearward the cam path unlocks the bolt, aligns it with openings in barrel extension
- momentum is keeping the carrier moving rearward.
Yeah, I understand the physical differences between the two systems. I just don't see where the bolt can go. I can see the gas inside the carrier making sure that the bolt stays forward during unlocking, but the same thing can be said about the firing pin spring on th HK bolt.
Yeah, I know, poor wording on my part perhaps. I didn't mean that unlocking starts when the carrier moves to the rear, I was trying to say that it was a result of the cam pin following the cam track, and the resulting rotation of both cam pin and bolt. If that makes sense.....I suck at english firearms nomenclature....sorry....
Gotcha. No apologies needed
Yeah, I understand the physical differences between the two systems. I just don't see where the bolt can go. I can see the gas inside the carrier making sure that the bolt stays forward during unlocking, but the same thing can be said about the firing pin spring on th HK bolt.
The bolt is held in place mechanically by the locking lugs during unlocking. It's also held in position within the carrier mechanically by the bolt cam. If anything holds the bolt in the forward most position in the carrier during extraction, it's inertia
While the gases do push against the bolt, they cannot push the bolt forward! Although the bullet has exited the barrel by the time the expansion chamber fills with enough pressure to start the carrier moving, the pressure in the bore is still greater. Also, the spent case has expanded enough to take up what little slack there was and resist any forward motion of the bolt.
It's also a fallacy to believe the AR does not have a piston. It's piston is part of the bolt and the carrier is a cylinder. The piston is inline with the bolt and bore, reducing muzzle climb induced by off center recoil forces and reciprocating mass.
The AR does not feature primary extraction, a mechanical camming action that starts extraction as the bolt begins unlocking. Many other designs do, such as the M14 and most bolt actions. Whether the AR uses the original inline piston or an aftermarket offset piston relocated to the gas block, it must rely on "brute force" to yank the spent cartridge from the chamber
^ That in a nutshell.
Another difference that I have not seen mentioned so far is that of reduced dwell time with a piston upper.
With a conventional DI system the gas enters the gas tube as the bullet passes the gas port, then has to travel all the way back the gas tube, enter the carrier and pressurize it before any carrier movement and extraction takes place.
This dwell allows the obturated case to shrink away from the chamber walls.
With a piston upper, as soon as the gas enters the gas port it can pressurize the small chamber and start the rearward piston movement immediately, which starts carrier movement and extraction pretty much right then. Obturation of the case may still be going on, which means the case is still gripping the chamber walls. Hence the extractor ripping through the rim, or just breaking.
Of course this is exacerbated when you have a 16" barrel and shorty gas system.
constructor
03-13-12, 19:32
There is no way the expansion chamber will pressurize enough to start moving the carrier before the bullet can travel from gas port to muzzle. The distance from gas port to muzzle is shorter than it is from gas port to carrier. Also, if I remember correctly, the velocity of the supersonic gases drop to subsonic after passing through the gas port. In the race between bullet to muzzle and gas to expansion chamber, the bullet wins every time.
When you see flames coming out of the chamber, it's for the same reason you see flames coming out of the muzzle. It's the spent gases igniting as they mix with the oxygen in the atmosphere
I'm guessing you are assuming the 15,000 psi gas behind the bullet moves at the same speed as the bullet which when passing by the port in the 7" location is apx 2400fps? You may want to do a little more research on that.
Ok are the flaming gases coming out of the muzzle high pressure or do you think they flow out all mellow like? If they shoot out of the ejection port that would say there's a little pressure behind them.
If you look at a pressure curve from a P. or PT system the 58,000 psi pressure is only while the bullet is in the first few inches by the time it passes the gas port it's 12-15000 psi and then around 8000 psi when the bullet exits depending on the caliber, powder and barrel length. after the bullet exits the pressure in the barrel drops to 0 very quick, there shouldn't be any pressure in the case so the walls of the case contract and allow it to be pulled from the chamber.
With a piston upper, as soon as the gas enters the gas port it can pressurize the small chamber and start the rearward piston movement immediately, which starts carrier movement and extraction pretty much right then. Obturation of the case may still be going on, which means the case is still gripping the chamber walls. Hence the extractor ripping through the rim, or just breaking.
Yeah, you would think that they didn't test this, huh? :rolleyes:
militarymoron
03-13-12, 19:57
With a piston upper, as soon as the gas enters the gas port it can pressurize the small chamber and start the rearward piston movement immediately, which starts carrier movement and extraction pretty much right then.
that's going to depend on the volume of that expansion chamber. you can't assume it's less than the volume of the gas tube and chamber inside the carrier, unless you know that it is.
Robb Jensen
03-13-12, 20:57
I'm guessing you are assuming the 15,000 psi gas behind the bullet moves at the same speed as the bullet which when passing by the port in the 7" location is apx 2400fps? You may want to do a little more research on that.
Ok are the flaming gases coming out of the muzzle high pressure or do you think they flow out all mellow like? If they shoot out of the ejection port that would say there's a little pressure behind them.
If you look at a pressure curve from a P. or PT system the 58,000 psi pressure is only while the bullet is in the first few inches by the time it passes the gas port it's 12-15000 psi and then around 8000 psi when the bullet exits depending on the caliber, powder and barrel length. after the bullet exits the pressure in the barrel drops to 0 very quick, there shouldn't be any pressure in the case so the walls of the case contract and allow it to be pulled from the chamber.
Exactly. On a DI AR as the gas enters the carrier key and the carrier the bolt receives forward pressure and the carrier rearward pressure as the carriers chamber fills with gas (at the same time since the bolt is the forward end of this chamber and the carrier is the rear of this chamber). When the chamber can no longer contain anymore the carrier moves rearward and this causes the bolt to unlock due to the track in the carrier (bolt connected to the carrier via cam pin) follows this track. By this time the chamber pressure has dropped so much that the empty case easily follows and stays attached to the bolt via the extractor. As the bolt reaches fully unlocked the gas rings pass by the two ports on the right side of the carrier and the gas is vented outside those ports.
One of the reasons for the H (3.8oz) and heavier CAR length buffers are to help delay the carrier from defeating the buffer and spring as soon as it would with a CAR (3.0oz) buffer, a heavier weight buffer is harder to defeat (simple physics). The CAR length gas guns are trying the move the bolt rearward faster than a longer rifle gas system does due to the shorter length and reduced gas system capacity. Because of the shorter gas system length the internal chamber pressure on the fired casing on a CAR gas gun is higher than it is on a mid length or rifle gas length gun, thus why the heavier buffers and stronger extractors help. Some of this can be controlled via the gas port in the barrel size. But it's still a timing issue, the system has to have enough gas to fill the gas system fast enough before the bullet leaves the barrel (dwell time). The stronger extraction devices (stronger extractor springs and O-rings) help keep the empty on the bolt due to this higher pressure on the empty case.
I'm guessing you are assuming the 15,000 psi gas behind the bullet moves at the same speed as the bullet which when passing by the port in the 7" location is apx 2400fps? You may want to do a little more research on that.
Ok are the flaming gases coming out of the muzzle high pressure or do you think they flow out all mellow like? If they shoot out of the ejection port that would say there's a little pressure behind them.
If you look at a pressure curve from a P. or PT system the 58,000 psi pressure is only while the bullet is in the first few inches by the time it passes the gas port it's 12-15000 psi and then around 8000 psi when the bullet exits depending on the caliber, powder and barrel length. after the bullet exits the pressure in the barrel drops to 0 very quick, there shouldn't be any pressure in the case so the walls of the case contract and allow it to be pulled from the chamber.
The velocity of the ejecta is faster than that of the bullet once it uncorks. In the formula to figure the free recoil of a rifle is a constant (if I recall correctly) is 4750 fps. However, the gas port reduces the flow of the gas increasing the time needed to pressurize the expansion chamber and get the carrier moving.
In order for the rifle to extract the spent case before the bullet clears the barrel, it would have to do so in fraction of a millisecond a that's all the time it takes the bullet to travel from gas port to muzzle
Let me add that until the bullet uncorks the gases in the bore are only traveling as fast as the bullet.
Robb, I'm pretty certain that the expansion chamber reaches operating pressures after the bullet uncorks. I believe the pressure and density of the residual gas is what the rifle is operating on
Robb Jensen
03-13-12, 21:38
Let me add that until the bullet uncorks the gases in the bore are only traveling as fast as the bullet.
Robb, I'm pretty certain that the expansion chamber reaches operating pressures after the bullet uncorks. I believe the pressure and density of the residual gas is what the rifle is operating on
It may.
But I think we would agree that there has to be enough gas volume fast enough to nearly fill the system with gas before the bullet leaves. If it didn't need to the gas port would be directly behind the barrel threads.
Once the bullet leaves the barrel most of the gas is going to follow the path of the bullet. Gases are much like electricity they follow the path of least resistance, bigger hole being the barrel not the gas port.
From what I remember Maj. Muth and LtCol Lutz taught me at Quantico was that a M16A2 needs 20% of the available gas to run properly. I don't know how that figures into DI CAR gas, piston, and DI guns with less barrel between the gas port and muzzle, but common sense tells me they would need more gas.
constructor
03-13-12, 22:33
Let me add that until the bullet uncorks the gases in the bore are only traveling as fast as the bullet.
Robb, I'm pretty certain that the expansion chamber reaches operating pressures after the bullet uncorks. I believe the pressure and density of the residual gas is what the rifle is operating on
Sure the gases are pushing the bullet so they can't travel faster than the bullet in the bore but they can go down the gas tube faster than they can push the bullet down the bore. As soon as the bullet leaves the bore the pressure in the barrel drops to 0. The dwell time between where the bullet passes the gas port and exits the barrel is when the gas tube and carrier (cylinder)fill with pressure and start moving. The inertia continues to move the carrier to the rear after the bullet has left the bore unless the rifle is over gassed where the carrier moves rearward and extracts the case too early. Cases landing ahead of 3 O'clock, breaking extractors and flames out of the ejection port are all signs of an over-gassed rifle.
The whole short stroke piston idea reminds me too much of smacking a hammer with another hammer. While the (debatable) reliability may be better I dont think the long term durability is as great with a piston ar.
I think the main advantage of a spring like pws is so you dont have to mess around with making sure the bolt is forward after disassembly to insert back into the upper. The looseness of the ruger sr556 bolt and how it flops around so much makes it a nuisance to align the cam pin back in the track.
Just keep in mind that when the bullet uncorks, pressure is escaping from the bore through both the muzzle and the gas port. Of course at that point, the majority will be escaping at the muzzle.
If you watch slow motion video of a bullet exiting a barrel, you will see that the gases vent under pressure for quite some time after the bullet uncorks.
I do not know how fast the expansion chamber pressurizes once the bullet clears the gas port. If the velocity of the gases drop to subsonic when passing through the port, they won't reach the expansion chamber until after the bullet exits. I'm uncertain how passing through a smaller port affects the supersonic flow of pressurized gases
Well, I am no gunsmith, but I have been doing some reading from several different sources.....in regards to the OP's question.
It seems that the most important factor in regards to avoiding excess strain on the bolt lugs, is the timing of the unlocking and cycling, and not how the gas pushes forward on the bolt or the impact of the oprod/tappet on the rod face on the carrier.
The HK416 is cycling faster than a DI, so has a heavier buffer to compensate (150 grams). I am sure this provides sufficient carrier lag, so that the pressure has subsided, and the casing has shrunk before the bolt is unlocked, ensuring proper extraction and ejection.
...It seems that the most important factor in regards to avoiding excess strain on the bolt lugs, is the timing of the unlocking and cycling...
You're correct and, as you pointed out, this fact nearly got lost in the shuffle
justin_247
03-14-12, 04:02
Well, I am no gunsmith, but I have been doing some reading from several different sources.....in regards to the OP's question.
It seems that the most important factor in regards to avoiding excess strain on the bolt lugs, is the timing of the unlocking and cycling, and not how the gas pushes forward on the bolt or the impact of the oprod/tappet on the rod face on the carrier.
The HK416 is cycling faster than a DI, so has a heavier buffer to compensate (150 grams). I am sure this provides sufficient carrier lag, so that the pressure has subsided, and the casing has shrunk before the bolt is unlocked, ensuring proper extraction and ejection.
This is correct.
Ridgerunner665
08-06-13, 23:58
I'm uncertain how passing through a smaller port affects the supersonic flow of pressurized gases
Ever use an air nozzle with compressed air? How about a pressure washer? Same physics either way
The smaller the hole, the higher the velocity...up to a point (density and mass come into play)...this is of course relative to the pressure behind the air (gas) to begin with.
Its actually the same whether its gas or fluid...a pin hole in a hydraulic line with enough pressure on it will spout a stream that will slice your arm off like a samurai sword...the same is true for high pressure gasses, but the range is shorter because the gas has less momentum (slows down in the atmosphere)
I know...I'm digging up old threads...but I'm reading to learn and thought what I posted here may be of some relevance.
I'm late the piston party...and playing catch up....and no, I'm not a physics professor...I'm a heavy haul truck driver, but my IQ is a bit higher than room temperature. ;)
EDITED TO ADD: A smaller gas port would also lower the volume of gas that went through during the firing process...because of the fixed amount of time that the pressure was there....this turned into a deep subject, LOL. I suspect many of you already knew the smaller port lowered the volume...we've all read about over gassing...which is exactly what some are saying the piston system amounts to because of the smaller chamber....if it is smaller...I honestly don't know yet.
At least give me credit for trying to understand, LOL...and I will understand before its over...and its not about me saying one is better than the other...its like the man said earlier...I just want to understand them both, particularly the piston though because thats what I have.
Anybody ever compared gas port sizes on any of the new piston guns to a DI? I may be wrong but this seems relevant to how hard the rod hits the carrier.....................I just remembered the adjustable gas block on my Sig 516, it vents the excess gasses out under the handgurads....so I guess that answers the overgassed question.
sinlessorrow
08-07-13, 00:36
Ever use an air nozzle with compressed air? How about a pressure washer? Same physics either way
The smaller the hole, the higher the velocity...up to a point (density and mass come into play)...this is of course relative to the pressure behind the air (gas) to begin with.
Its actually the same whether its gas or fluid...a pin hole in a hydraulic line with enough pressure on it will spout a stream that will slice your arm off like a samurai sword...the same is true for high pressure gasses, but the range is shorter because the gas has less momentum (slows down in the atmosphere)
I know...I'm digging up old threads...but I'm reading to learn and thought what I posted here may be of some relevance.
I'm late the piston party...and playing catch up....and no, I'm not a physics professor...I'm a heavy haul truck driver, but my IQ is a bit higher than room temperature. ;)
EDITED TO ADD: A smaller gas port would also lower the volume of gas that went through during the firing process...because of the fixed amount of time that the pressure was there....this turned into a deep subject, LOL. I suspect many of you already knew the smaller port lowered the volume...we've all read about over gassing...which is exactly what some are saying the piston system amounts to because of the smaller chamber....if it is smaller...I honestly don't know yet.
At least give me credit for trying to understand, LOL...and I will understand before its over...and its not about me saying one is better than the other...its like the man said earlier...I just want to understand them both, particularly the piston though because thats what I have.
Anybody ever compared gas port sizes on any of the new piston guns to a DI?
Every op rod AR I have seen cycles faster than a standard AR system.
The HK is a great example, to counter that they use a 8lb action spring and a 5.3oz buffer which is at the H3 level. Even then it still cycles faster than a Colt 6920 which comes with a H2(4.3 oz buffer) and a 5.5-6lb action spring.
Ridgerunner665
08-07-13, 00:55
Every op rod AR I have seen cycles faster than a standard AR system.
The HK is a great example, to counter that they use a 8lb action spring and a 5.3oz buffer which is at the H3 level. Even then it still cycles faster than a Colt 6920 which comes with a H2(4.3 oz buffer) and a 5.5-6lb action spring.
Agreed...
It has to be faster...because the work starts sooner...the piston system doesn't have to wait for the gas to get to the carrier/bolt.
So now we're back to the unlocking thing...where you were when I got here, LOL. Before I can comment on the unlocking timing I'm gonna have to find out exactly how it works in the DI system...I'm not sure which side of the rings the gas enters the carrier on...nor have I ever given any thought to the actions the bolt goes through during firing.
Got some studying to do...but its bed time for now...gotta roll a long way tomorrow, delivering to Ft. Stewart...and I'm in Indiana.
sinlessorrow
08-07-13, 01:02
Agreed...
It has to be faster...because the work starts sooner...the piston system doesn't have to wait for the gas to get to the carrier/bolt.
So now we're back to the unlocking thing...where you were when I got here, LOL. Before I can comment on the unlocking timing I'm gonna have to find out exactly how it works in the DI system...I'm not sure which side of the rings the gas enters the carrier on...nor have I ever given any thought to the actions the bolt goes through during firing.
Got some studying to do...but its bed time for now...gotta roll a long way tomorrow, delivering to Ft. Stewart...and I'm in Indiana.
The AR does not use a DI system, though that name has just stuck.
It is an inline piston, the gas that pushes the bullet down to bore is siphoned off and sent down the gas tube, which travels into the carrier key. From the carrier key it travels down into the expansion chamber that is formed by the carrier and bolt tail(behind the gas rings).
When the gas expands it pushes against the bolt tail which has formed a air tight seal thanks to the gas rings, since the bolt cannot move the carrier is instead sent backwards, as the carrier moves rearward the cam pin is engaged and unlocks the bolt.
Once the bolt is fully unlocked the gas inside the expansion chamber is exhausted out of the 2 holes in the side of the carrier and the BCG travels rearward, ejects the brass, and chambers a new round.
If you do not know a DI system is similar in that is has a gas tube, but in a DI system the gas that travels down the gas tube is sent into a small shallow cup in the front of the carrier, when the gas strikes this cup the force is what cycles the action leaving all the gas to be expelled into the action/air.
If you do not know a DI system is similar in that is has a gas tube, but in a DI system the gas that travels down the gas tube is sent into a small shallow cup in the front of the carrier, when the gas strikes this cup the force is what cycles the action leaving all the gas to be expelled into the action/air.
Just like in the French MAS49.
Ridgerunner665
08-07-13, 01:20
The AR does not use a DI system, though that name has just stuck.
It is an inline piston, the gas that pushes the bullet down to bore is siphoned off and sent down the gas tube, which travels into the carrier key. From the carrier key it travels down into the expansion chamber that is formed by the carrier and bolt tail(behind the gas rings).
When the gas expands it pushes against the bolt tail which has formed a air tight seal thanks to the gas rings, since the bolt cannot move the carrier is instead sent backwards, as the carrier moves rearward the cam pin is engaged and unlocks the bolt.
Once the bolt is fully unlocked the gas inside the expansion chamber is exhausted out of the 2 holes in the side of the carrier and the BCG travels rearward, ejects the brass, and chambers a new round.
If you do not know a DI system is similar in that is has a gas tube, but in a DI system the gas that travels down the gas tube is sent into a small shallow cup in the front of the carrier, when the gas strikes this cup the force is what cycles the action leaving all the gas to be expelled into the action/air.
Yeah...I remember that from reading through this thread earlier...but to get a grasp on it I think I'm gonna have to have a BCG in my hands...I've tried just running through in my head but the part where the bolt and the carrier start moving as individuals gets hard to follow without visual aids, LOL.
I'll be home Friday...I have visual aids there.
sinlessorrow
08-07-13, 01:28
Yeah...I remember that from reading through this thread earlier...but to get a grasp on it I think I'm gonna have to have a BCG in my hands...I've tried just running through in my head but the part where the bolt and the carrier start moving as individuals gets hard to follow without visual aids, LOL.
I'll be home Friday...I have visual aids there.
The bolt does not move aside from rotating. The only part that moves is the carrier, which as it travels rearward the cam pin engages its tract and this movement of the cam pin is what rotates the bolt unlocking it.
Other than that rotation the bolt stays stationary.
Ever use an air nozzle with compressed air? How about a pressure washer? Same physics either way...
Yes, I have- plenty of times. Bernoulli's Principle tells us that running a flow through a restriction will result in a drop in pressure and rise in velocity, so long as we're dealing with subsonic velocities. But the expanding gas inside a rifle is supersonic- and that changes the rules. When supersonic gas flows through a restriction (like the gas port) the velocity drops to the speed of sound. What the speed of sound is under these conditions, I haven't found out yet.
But we do know that the projectile leaves the muzzle before the gases can overcome the inertia of the reciprocating mass. You can see it for yourself when studying slow motion videos of gas operated firearms being fired
The bolt does not move aside from rotating. The only part that moves is the carrier, which as it travels rearward the cam pin engages its tract and this movement of the cam pin is what rotates the bolt unlocking it.
Other than that rotation the bolt stays stationary.
You might want to think it through a little further before making that assertion :)
My Adams Arms piston system has no carrier tilt. The carrier is all one piece and has no signs of wear. Pistons are superior to DI in every way. Pistons operate combustion engines ... lol since the Model T and earlier in Germany. The piston concept is the best method of harvesting energy into mechanical movement.
Arguing against pistons is like trying to reinvent the combustion engine.
sinlessorrow
08-07-13, 14:48
My Adams Arms piston system has no carrier tilt. The carrier is all one piece and has no signs of wear. Pistons are superior to DI in every way. Pistons operate combustion engines ... lol since the Model T and earlier in Germany. The piston concept is the best method of harvesting energy into mechanical movement.
Arguing against pistons is like trying to reinvent the combustion engine.
Its a good thing stoner was a genious and developed a proprietary inline piston system.
.46caliber
08-07-13, 14:48
You are now entering...the twilight zone.
Sent from my Nexus 4 using Tapatalk 2
My Adams Arms piston system has no carrier tilt. The carrier is all one piece and has no signs of wear. Pistons are superior to DI in every way. Pistons operate combustion engines ... lol since the Model T and earlier in Germany. The piston concept is the best method of harvesting energy into mechanical movement.
Arguing against pistons is like trying to reinvent the combustion engine.
First, the AR already has a piston. If you'd sit down and apply critical thinking to figuring out how it works using the laws of physics instead of simply repeating internet memes, you'd understand that.
Second, it's turbine engines that power large aircraft around the globe and rocket engines that power our space program.
Third, just because it's rocket science, doesn't mean it's complicated. Launching a bullet using gunpowder uses the same principles
elephantrider
08-07-13, 15:19
My Adams Arms piston system has no carrier tilt. The carrier is all one piece and has no signs of wear. Pistons are superior to DI in every way. Pistons operate combustion engines ... lol since the Model T and earlier in Germany. The piston concept is the best method of harvesting energy into mechanical movement.
Arguing against pistons is like trying to reinvent the combustion engine.
Carrier tilt? This thread isn't even about that. Did you read the thread title, or any of the preceding posts? This discussion is about bolt unlocking.
A big endorsement for pistons from the guy who a few weeks ago couldn't figure out why his piston gun wasn't working.:rolleyes:
Even with DI I love the AR system. I'm still curious if anything can replace it or how long it will take. Small arms will be what in 200 years?
My Adams Arms piston system has no carrier tilt. The carrier is all one piece and has no signs of wear. Pistons are superior to DI in every way. Pistons operate combustion engines ... lol since the Model T and earlier in Germany. The piston concept is the best method of harvesting energy into mechanical movement.
Arguing against pistons is like trying to reinvent the combustion engine.
Weren't you told by multiple mods here recently to read more & post less?
Sent via Tapatalk
sinlessorrow
08-07-13, 16:36
Even with DI I love the AR system. I'm still curious if anything can replace it or how long it will take. Small arms will be what in 200 years?
Caseless/CTA unless we develop mini rail guns, then laser rifles.
Though this is incredibly off topic. This also was not a Inline vs op rod operating system discussion, but a discussion about force difference when the bolt unlocks.
My Adams Arms piston system has no carrier tilt. The carrier is all one piece and has no signs of wear. Pistons are superior to DI in every way. Pistons operate combustion engines ... lol since the Model T and earlier in Germany. The piston concept is the best method of harvesting energy into mechanical movement.
Arguing against pistons is like trying to reinvent the combustion engine.
The recip IC engine is a terribly inefficient device. That is why the turboprop and turbofan replaced the recip in aviation.
My Adams Arms piston system has no carrier tilt. The carrier is all one piece and has no signs of wear. Pistons are superior to DI in every way. Pistons operate combustion engines ... lol since the Model T and earlier in Germany. The piston concept is the best method of harvesting energy into mechanical movement.
Arguing against pistons is like trying to reinvent the combustion engine.
The "DI" AR is a piston gun. Always has been. Ever look at the bolt tail?
Ceaseless ammunition failed already, just ask HK. I think ceaseless is a waste. Rail guns seem likely.
The OP asked about force and wear, I'm just pointing out a piston and machine parts can handle the force because we see the designs on a greater scale.
Ceaseless ammunition failed already...I think ceaseless is a waste...
I don't care if it's a waste or not, I'd love to have ceaseless ammunition! I'd never make to work and it'd be amazingly hard on the barrels, but I'd be happier than a tornado in an Occupy Camp!
I'm just pointing out a piston and machine parts can handle the force because we see the designs on a greater scale.
That's absolutely not what you're doing, when you swipe with drek like "superior system" using a brush as broad as the one you selected. We've enough existing piston vs. DI argument threads where that assertion is objectively dispelled, starting with the monetary aspect in regard to the user that's buying his own crap...just for starters.
And, I don't care if it's caseless OR ceaseless, the non-productive derail ends right here. Pretty please, with sugar.
However awesome the latter would undoubtedly be....
Ridgerunner665
08-11-13, 23:57
OK...I've been sitting around working the BCG back and forth trying to get my brain wrapped around the firing process.
After much deep thought...I think how "violent or early" the extraction is depends entirely on the timing of the cam pin in the groove in the upper and the length and angle of the cam pin slot in the carrier. I believe its the cam pin itself that is absorbing the most punishment...not the bolt lugs...the cam pin is taking a pretty hard lick when it hits the front of the slot in the carrier...but this should be the same for a DI setup so I don't think its anything new.
If the angle and length of the slot in the carrier and the track in the upper are correct everything lets go when it should . The lugs on my bolt do show very minor wear (just to the finish, and its has barely worn that), but all of them are the same...meaning, they are even and well mated to the extension when locked in. The timing of my Sig 516 is such that the carrier moves appx. 1/8" before the bolt starts rotating...this is the "moving individually" I was referring to, I was "hoping" it was there...the bolt seems to rotate all the way to the unlocked position before the carrier starts pulling it to the rear...the only way the bolt could be rotating while still under load is if the carrier started pulling it as soon as the op rod hit it...but thats not how it works...the carrier has to move that 1/8", which don't take long...but thats long enough for the bullet to exit the barrel, let the pressure drop, and let the brass to let go of the chamber walls....bolt thrust has done been and gone by this time.
The 516 bolt does have radiused edges on the side that matters, its made that way, not worn that way...I can tell because of the finish on them.
AR timing is something I had never considered at all...reminds me of when I first dove into 1911's...and that is one firearm I do understand (but it didn't happen without a lot of effort, and some help)
In a nutshell...Violent/early extraction issues are possible if everything is not properly timed...but if there was such a timing issue the rifle would probably not run (FTE, short stroke) and would chew up bolt lugs faster than you could change bolts...it wouldn't take long to see the problem.
I'm not betting the farm that I am correct on this...but it looks right to me.
Ok, turd in the punch bowl.
What about a steel case with much more taper?
Would not more/longer pressure of the bolt/piston be needed to smooth unlocking?
Timing and tensions are not mutually the same. What do you propose?
Old thread, but a few things:
With the M16 gas tube design
1) From experimental measurements, the time of the first pressure rise in the carrier cavity (the cylinder) is about .25 ms after the bullet passes the gas port. The time of peak pressure is about .75 ms later.
2) From experimental measurements, the pressure in the carrier cavity at the start of unlocking is around 1700 psi, the chamber pressure at the start of unlocking is around 2500 psi. Calculating the respective forces generated by the respective areas shows the forces to be roughly equal. If there is a force imbalance pushing the bolt forward, it would show up as spent cases being shorter than the chamber headspace. Since this is not the case (at least I have never heard of it), it would seem the nets force on the bolt is always against the barrel extension lugs, or the force is well below the force required to resize the case. Either way, there is no 'assisting extraction' from the gas pressure force on the bolt.
3) From experimental measurements, the bolt begins to unlock about 1 ms after the bullet has passed the gas port, and complete unlocking takes about .75 ms. Bullet exit is about .25 ms after passing the gas port.
4) Models based on these experiments and others indicate that the timing of a barrel mounted piston with a diameter of .312" will be approximately the same as that of a gas tube design. The only major difference will be the locking lugs will not see a forward bolt thrust. However, the chamber pressure at the start of unlocking is only 2500 to 3000 psi, with a thrust of 270 pounds. The pressure at completion of unlocking is 1000 psi with an associated thrust of 100 pounds, so lug forces are minimal.
5) The piston thrust from a barrel mounted piston are intentionally lower to keep the initial acceleration of the carrier down so unlocking will occur at the same time as the gas tube design, however the stroke of the piston is double that of the gas tube design (.750" versus .325"), this leads to higher final carrier velocity and higher cyclic rates.
With the M4 gas tube design:
6) From experimental measurements, the gas tube and carrier cavity pressure are roughly the same as with the M16, but everything occurs about 0.25 ms sooner. And, the chamber pressures (and bolt forces) during unlocking are about 75% higher.
7) Even though the moving mass is less (lighter buffer), the timing of events during the first .325" of carrier travel is roughly the same, however, the final carrier velocity is higher.
Steel cases:
The difference between steel and brass cartridge cases is steel does not obturate during peak pressure as well or spring back as much as brass after the peak chamber pressure. This means the force required to pull the case from the chamber (extraction force) is higher. Extraction occurs after the carrier cavity has vented, so there is little gas pressure used during this phase of the operational cycle, bolt carrier momentum is the primary energy source for extraction. This being the case, you either have enough energy to complete extraction with sufficient left over to complete ejection, feeding and locking, or you don't. A higher case taper makes extraction easier, as there is less sliding contact between the case and the chamber wall.
In any case, extraction has practically no influence on unlocking in the AR bolt design as the bolt must rotate around 22 degrees (approximately .020" from completely unlocked) before it can start to move the rear and extract the case, and the extractor slides around on the rim during unlocking. This is because the locking faces of the barrel extension as well as the locking faces of the bolt are perpendicular to the axis of the bore, as long as any portion of the bolt lugs are in contact with the extension lugs, rearward motion is prevented. (There is a .020" chamfer on both the bolt and extension lugs, primarily to ensure that no burrs remain on the corners.)
The less effective obturation of steel cases, similarly, plays no part during unlocking. It does, however, play a part in extraction, but only because more gas and fouling residue is allowed to get in between the outer case and the chamber wall. The residue on the case is no problem as it is extracted and ejected, however, there may be increased fouling in the chamber, eventually leading higher required extraction forces, due to sticking.
EDIT:
All the stuff about barrel mounted pistons (aka "piston systems") is based on the .312" diameter piston, and associated internal passage geometry used in the Adams Arms piston design. Enlarging the piston diameter, increasing or decreasing the gas passage diameters, or changing the weight of the cylinder/operating rod, will change everything, obviously.
Thanks for all the Data! Very much a good read, especially for someone like myself, a piston owner.
My Adams Arms will feed everything really well until the chamber starts getting dirty. Finally had to ditch buying Tula as the steel wouldn't extract well once it got dirty. Otherwise, I have never had a hangup from brass before.
After having to manually knock out 4 tula in a row, I decided to try some of my LC M855, fed just fine and worked without issues. After reading your post, I think I have a pretty good understanding of most of the "why" behind that.
As always, thanks for sharing; this bit of advisement will surely be beneficial in the future.
With the Adams Arms, you could have just switched over to the big hole and kept running Tula.
The big hole?
Im not sure i quite understand.
I have a 4 position piston block.
1. No gas, have to manually cycle between each shot.
2. Supressed, reduces pressure due to backflow from can.
3. Unsupressed, or "open". This was what it is ran from.
4. Unlock piston and remove for cleaning.
As mentioned, it runs fine for multiple magazines, but will stop feeding reliably at somepoint. Very well could be extractor issues, but i dont have more than 2k rounds through it.
I hope I dont come off cross, not the case at all, just not sure what you mean.
The big hole?
Im not sure i quite understand.
I have a 4 position piston block.
1. No gas, have to manually cycle between each shot.
2. Supressed, reduces pressure due to backflow from can.
3. Unsupressed, or "open". This was what it is ran from.
4. Unlock piston and remove for cleaning.
As mentioned, it runs fine for multiple magazines, but will stop feeding reliably at somepoint. Very well could be extractor issues, but i dont have more than 2k rounds through it.
I hope I dont come off cross, not the case at all, just not sure what you mean.
Oh, you're already running it on the "big hole" setting. Mine runs acceptable on the "suppressed setting" without a supressor, so I have the "unsuppressed" as a back-up if I get fouling problems.....
Oh! I understand what you're referencing then. :)
I know the rifle will operate just fine on supressed with the M855 and such, when it comes to the Tula, I, honestly, haven't even tried it on the lower setting.
I still have a few hundred on these Tula to blow through (looking back, I can't even imagine how many payday weekends at 3 boxes max per trip I had to even make to accumulate this much ammo) and will see if it was maybe just a particular lot.
I didn't really experience these issues initially, but it's been mostly fed LC M855 from an old Bandolier can as that is my current "go-to-round" as I have that stockpiled even heavier than the Tula.
The tula will work, even in a pinch, for awhile. Even after fouling up, I am sure a quick patch through would reopen it for many rounds. Not sure if my tolerances are just that tight on my AA15, or if the ammo I am using is simply that dirty. Either way, after repeated stuck cases, I decided it wasn't worth all the effort. Especially when the case lip rips off instead of pulling the round during a slam drill.
So I have a quick question regarding piston operated uppers.
In a normal D.I. upper, as gas is imparted into the gas tube, it travels down through the gas key, into the cavity in the carrier, and pushes the bolt itself forward. This eases extraction while also thrusting the BCG rearward. So essentially the gas is used to force the carrier rearward, bolt forward, rotate and unlock the bolt from the breach, then extract the round as it travels rearward.
However, in a piston system, the gas key is replaced with just a flat piston key which the short stroke rod impacts and forces the BCG rearward... so there is no forward motion of the bolt to help unlock and ease the stress on the lugs right? It just rips it right out... which to me sounds very detrimental to the system... it extracts the round before it may be at the optimal size to extract ( forward motion of the bolt by a D.I. system provides sufficient time for the case to cool and shrink) and it just shoves the carrier to the rear and basically just rips the lugs out, causing them to sheer against the extension lugs until they eventually unlock and allow the bolt to leave.
Basically piston systems just use brute force to rip the bolt/shell out of the chamber... seems to me like carrier tilt would be a drop in the bucket compared to the stresses that type of action has on the rifle, so the various retro-fit kits available would be about as beneficial as using a filled diaper to lube your gun...
Is this assessment correct?
My question is this:
Do you think the bled gas system pressure from the gas port is anywhere near the chamber pressure, to the point that gas system pressure in the internal expansion system would overcome even residual chamber pressure?
That's a tough sell from a physics perspective, which is a nice way of saying, "No way."
Op-rod driven designs have to compensate for the increased port diameter and gas volume by buffering and springing up in the recoil system, in order to try to tame unlocking shear, early extraction of obturated cases, and violent ejection.
ETA: I see lysander covered this in great detail already.
...bolt carrier momentum is the primary energy source for extraction...
EDIT:
The above is technically correct, but I forgot to mention there is that 1000 psi residual chamber pressure assisting extraction, and it is pretty important.
Testing has shown that during extraction, the extractor jumps off the case rim, if the residual chamber pressure is too low, or absent, the bolt will move to the rear and loose contact with the case (by the forward push of the ejector, and exacerbated by a dirty chamber or dinged cases). When the extractor returns to its normal position, it misses grabbing the rim and you have a failure to extract malfunction.
This was proven by configuring two M4 Carbines, one above the other with the top M4 gas tube inserted in the lower M4's bolt carrier. The lower M4 was fired, and without its gas system hooked up, it did not cycle but left the spend cartridge in the chamber. Then the top M4 was fired and the gas from the top M4 was used to cycle the lower M4. The lower M4 suffered failures to extract....
www.dtic.mil/ndia/2003smallarms/din.ppt
The lower M4 failed to extract with a normal bolt.
What I don't see listed is which extractor spring configuration was used.
The weaker rifle spring (which is generally insufficient) or an upgraded spring / o-ring.
When the Ejector was removed, the lower M4 extracted properly.
So the residual chamber pressure must be sufficient to overcome the effect of ejector tension.
Additionally, we've done some tests with the Extractor removed.
https://www.m4carbine.net/showthread.php?173514-No-Extractor
Some gas system configurations actually "self extracted" due to residual pressure.
The report is dated May 2003, so the extractor spring used was probably whatever was standard on M4s then.
Oh, and I found an old report on steel cases done back in the early 1970s. One of the curious things they noted during the endurance testing was steel cases tend to self-extract in the event of a broken extractor, where brass cases do not. So, the phenomenon may be related to case material, as well as port location relative to the end of the barrel....
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