AR- Piston or Direct Impingement?

[montrala]

Originally Posted by Arctic1

Is there a way of calculating the actual effect of this, and compare it to an inline piston/DI system? The piston and oprod of an HK416 weighs 119 grams, moves maybe an inch during cycling and is located 2,2cm over the centerline of the bore. Don't know the velocity...

There is one very important factor here, that makes calculations way more difficult. In HK416 piston and op-rod stop their travel before BCG does and piston with op rod travel forward and come to full stop before BCG. This counteracts some of torque generated initially by rearward movement of piston and op-rod and split torque impulses in time. Actually this is why Mr. Dragunov decided to use spring loaded short stroke system in his "precision rifle" - to reduce muzzle rise by changing timing of moving parts and reduce mass of those parts. Later on Russians also constructed prototype assault rifle (competed against AN-94), that practically canceled recoil by meant of timing masses moving in different directions (but was even more complicated than AN-94).

Personally I do not think that in HK416, it's piston and op-rod that are very light and move very close to barrel line, muzzle rise is more than in in-line (DI) piston AR-15 due to different operation system. However sharper recoil impulse, that HK has (rifle was designed as 10" system for use in harsh conditions) makes it harder to control by some used to differently working rifles.

Personally I do not have this experience, but users that switched from KAC SR-16 to HK416 actually feel that HK is easier to control in full-auto fire (however this can be due to higher overall mass of HK416), so no easy answers here.

For me situation that AR15 pattern rifles (with patent protection expired long ago) are only military rifles with "DI" system, while all other use either short or long stroke "piston" system (even some AR15 patterned rifles) is some kind of a hint. When some "HSLD" units over the world adapt AR15 patterned weapon systems, it is due to it's vastly superior ergonomic and flexibility of configuration, not due to "DI" system. For military purposes AR15 patterned weapons have sufficient accuracy with either operating system.

[Jippo]

Originally Posted by montrala

Personally I do not think that in HK416, it's piston and op-rod that are very light and move very close to barrel line, muzzle rise is more than in in-line (DI) piston AR-15 due to different operation system. However sharper recoil impulse, that HK has (rifle was designed as 10" system for use in harsh conditions) makes it harder to control by some used to differently working rifles.

You make some good points Montrala. I just wanted to add to this one with some of my experiences with the MR308. I have the 20" barrel version which uses, interestingly enough, the same length gas system as the 16" variant does. I have very little experience with other AR10 pattern guns, but I do find the BCG of the MR very heavy and the action spring is pretty strong as well. I believe they did design a lot of excess into the system to assure reliability in all conditions.

Interestingly enough the rifle recoils very smoothly, much like a bolt action .308. Thus I was not very surprised when I saw the video comparison posted the first time. Also all the people that have fired the gun have very uniformly commented on the soft shooting action it has, many of them very experienced riflemen.

[MistWolf]

Originally Posted by Clint

One other thing to consider is how the gas is delivered.

We've covered that the overall level of gassing or total impulse is important.

What also matters is the time duration over which that impulse is delivered.

Quick, high force or slow, low force.

The main driver is dwell time or the time from when the bullet passes the gas port until it exits the muzzle.

One of the supposed benefits of external piston systems is the gas block can be near the muzzle.

This requires the total impulse to be delivered in a very short time producing very high forces.

US Army research shows with the Garand and the M14, the bullet is several feet from the muzzle before the piston even begins moving. Considering the dwell time of the bullet going from the gas port to the muzzle is less than half a millisecond, I doubt it has much affect on the operation of the action at all. I believe venting time is the greatest influence. The fact that suppressor use increases venting time and increases BCG velocities supports this theory

[Clint]

Originally Posted by MistWolf

US Army research shows with the Garand and the M14, the bullet is several feet from the muzzle before the piston even begins moving. Considering the dwell time of the bullet going from the gas port to the muzzle is less than half a millisecond, I doubt it has much affect on the operation of the action at all. I believe venting time is the greatest influence. The fact that suppressor use increases venting time and increases BCG velocities supports this theory

Yes, "blowdown" time does contribute to the total impulse.

Suppressors do extend this blowdown time, as does barrel length.

On modern designs, I don't believe blowdown is the greatest contributor.

Actual pressure traces would show this.

The Garand system gas port is very near the muzzle of a very long barrel using a very large piston at low pressure.

I wouldn't be surprised if it operated mostly on Blowdown.


Consider how much smaller the gas port is on a 16" carbine vs a 16" middy. Blowdown time is identical and dwell is reduced.

[MistWolf]

I see your point, Clint. I don't have the dwell chart with me at the moment, but the time difference between the carbine and the middie is very short. Is it long enough to make a difference? The gases will be slowed to subsonic speeds after passing through the gas port. This ensures teh bullet will be long gone before the gas reaches the BCG.

The overall blow down time from ignition until the system drops to atmospheric is the same with either the carbine or middy when both barrels are of the same length. But blow down time from the time the bullet passes the gas port until the system drops to atmospheric is longer with the carbine. I do not know how much longer.

I admit my "Blow Down Theory" is just a theory, but the affect a suppressor has supports it

[MistWolf]

Came across an informative video by Brownell's that explains how an AR works. The only error is claiming the gases move the bolt forward. They do not. Hope this helps explain things
http://www.brownells.com/.aspx/lid=1...arn/?source=CJ

[sinlessorrow]

Originally Posted by MistWolf

Came across an informative video by Brownell's that explains how an AR works. The only error is claiming the gases move the bolt forward. They do not. Hope this helps explain things
http://www.brownells.com/.aspx/lid=1...arn/?source=CJ

Goodfind, thats a very good video.

[afroney]

Originally Posted by tpelle

We can argue this until the cows come home and neither side will convince the other. The point is, both systems can be made to work.

When Eugene Stoner designed the so-called DI system (So-called because it is NOT the same as a true DI system as employed on the Llungman rifle) he did so because he was working to a "maximum weight with 20-rounds of ammunition" spec. Later, when he had the opportunity to design what he felt was the rifle that the military REALLY needed, he designed the AR18 with a piston.

Simply taking an AR and making it a piston-driven rifle by changing the gas key to a solid lump and adding a piston to push against it is inherently problematical because, unless you come up with a way to guide the bolt carrier, you end up with bolt tilt.

Piston systems do not eliminate fouling, they just move it to a different place.

DI systems do not, in and of themselves, cause rifles to fail due to heat or fouling. Just look at the Colt's video of the full-auto-to-destruction test of the M4 carbine. The rifle didn't fail because of the DI system, it failed at 535 rounds of continuous full auto fire because the barrel melted.

On the other hand I have some experience with a rifle with a piston system that is very similar to many of the ones being retrofitted to AR15s - the FAL. I personally have seen FALs with the gas piston so fouled that, after removal of the gas plug, you had to shotgun the rifle open, remove the bolt assembly and dust cover, and poke something into the gas piston hole in the receiver to push on the rear end of the gas piston to free it up so you could take the piston out to clean it.

In the end I think it comes down to these three points:

1. Buy what you want.

2. DI works just fine in an AR, and that's what I buy. I've yet to personally have any issue with my rifles in the usage that I give them that was a consequence of DI. But I'm buying on the tail end of 40+ years of a maturation process that DI has gone through, so I'd expect it to be reliable.

3. But if you want to have a piston AR, buy that instead. But be ready to accept the consequences if nothing is done to additionally guide the bolt carrier against the off-center thrust, and maybe you have to do a little R&D on the less-than-mature system to get the dwells, pressures, bolt velocity, etc. right.

Solid points. As a DI and Piston owner, I would agree.

I own one of LMT's piston carbines. I've put somewhere in the 12-15k rounds through it (mostly cheap russian ammo). The advantages I've noticed over my DI weapons are as follows:

- The LMT seems to be less picky about ammo than my DI weapons. I've noticed significantly less malfunctions with the lower powered steel cased ammo (Tula, Uly, or old Wolf). With the LMT piston, I can also mix brass and steel cased in a mag without having a case stick. My DI weapons will occasionally choke if I do the same.
- Less lubrication burn off (specifically using CLP). (although I don't shoot suppressed or auto). I've found Mobil 1 stays put on both systems.
- Slightly easier to clean. Less carbon to scrape (If you are one of those white glove types.)

The disadvantages I've seen to the system

- Carrier tilt. I lost a buffer retainer pin after the bolt carrier damaged the extension tube enough to spit the buffer retaining pin out. The pin landed in the FCG and locked up the weapon. An anti-tilt buffer solved that issue. I actually have pictures of this in the piston forums on AR15.com
- Like your FAL example, if I shoot the LMT enough without cleaning the piston system, it becomes increasingly difficult to pull the piston/plug assembly out. I shot over a case of Uly without cleaning (which is filthy ammo) and had to pull the piston plug out with the pliers on my Leatherman. Amazingly, the weapon was still cycling. I now spray a shot of CLP into the piston plug, before pulling it out.
- Some proprietary parts. The piston, gas block, barrel, and bolt carrier are unique to LMT.

Its a toss up to what I would select if I bought another AR. I do prefer the piston AR for high volume blasting due to the advantages above. However, I have no doubts that a DI AR will run just as well as a piston AR, given ample lubrication and high quality ammo.

[sinlessorrow]

I know alot of times in the debate about how the DI system works alot seem to think the expanding gas pushes the bolt forward. This is false, I found this very good cut away that shows that there is no room for the bolt to be pushed forward in the extension. The cutaway also shows how the gas is sent behind the bolt and gives an idea of how the piston in the system works.

[militarymoron]

Originally Posted by sinlessorrow

I found this very good cut away that shows that there is no room for the bolt to be pushed forward in the extension.

if there wasn't any space there wouldn't be enough clearance for the bolt to rotate and lock into the barrel extension. it's not like a slip fit bushing. it's a pretty loose fit, and this allows front-back movement of the bolt when it's locked. this is what they're talking about - a minute amount of movement - smaller than you can see in that photo. i'm talking about a few thousandths of an inch.

[Failure2Stop]

Originally Posted by sinlessorrow

I know alot of times in the debate about how the DI system works alot seem to think the expanding gas pushes the bolt forward. This is false, I found this very good cut away that shows that there is no room for the bolt to be pushed forward in the extension. The cutaway also shows how the gas is sent behind the bolt and gives an idea of how the piston in the system works.

The system most certainly exerts force forward on the bolt.

Typos brought to you via Tapatalk and autocorrect.

[sinlessorrow]

Originally Posted by Failure2Stop

The system most certainly exerts force forward on the bolt.

Typos brought to you via Tapatalk and autocorrect.

There may be force put on the bolt from the rear but there will not be any movement forward of the bolt like alot seem to think when they bring up the expanding gases, I guess I could have explained that a bit better. Alot of times you see people mention that when the gas expands its puts forward pressure on the bolt pushing it forward and reducing stress on the bolt lugs. This is not true.

I also notice alot of times on bolts the front part of the bolt lugs will have finish wear, I would reckon this comes from the locking and unlocking of the bolt slightly rubbing against the barrel extension, correct me if I am wrong here?

[MegademiC]

Originally Posted by sinlessorrow

There may be force put on the bolt from the rear but there will not be any movement forward of the bolt like alot seem to think when they bring up the expanding gases, I guess I could have explained that a bit better. Alot of times you see people mention that when the gas expands its puts forward pressure on the bolt pushing it forward and reducing stress on the bolt lugs. This is not true.

I also notice alot of times on bolts the front part of the bolt lugs will have finish wear, I would reckon this comes from the locking and unlocking of the bolt slightly rubbing against the barrel extension, correct me if I am wrong here?

Even if it doesnt move, there is pressure pushing foreward on the bolt. You have the force of the main propelent pushing back, and this pushing foreward(to a lesser extent). Some of the forces cancel reducing the overall value of the rearward force vector. This will reduce friction on the lugs. If a sled is on the ground and you lay on it, it doesnt move, but there is much more static friction to overcome, and after that there is still more friction. This will cause the sled to get a hole much quicker.

[sinlessorrow]

Originally Posted by MegademiC

Even if it doesnt move, there is pressure pushing foreward on the bolt. You have the force of the main propelent pushing back, and this pushing foreward(to a lesser extent). Some of the forces cancel reducing the overall value of the rearward force vector. This will reduce friction on the lugs. If a sled is on the ground and you lay on it, it doesnt move, but there is much more static friction to overcome, and after that there is still more friction. This will cause the sled to get a hole much quicker.

This is true, there is gas pushing the bolt forward but it does not "move" forward like is generally said.

Not only is there practically no room(maybe a thousands of an inch as MM said, though I am still doubtful that little bit would even make any difference.) and the rearward force from the chamber there is no way for the bolt to actually move forward.

[MistWolf]

In order for the gases to move the bolt forward, there must be greater pressure inside the BCG than inside the bore. Pressure inside the BCG will either be less than or equal to the pressure inside the bore. Never greater. The pressure inside the BCG cannot move the bolt forward. What happens is that the pressure inside the bore simply drops enough that it's no longer exerting back thrust

[Treiz]

The cutaway also shows how the gas is sent behind the bolt and gives an idea of how the piston in the system works.

I disagree with the characterization of Stoner's design as a Piston system. Yes it does involve a stationary piston as stated in his patent, but the system does more closely resemble an impingement system. The gases still transfer their energy to the carrier, as with the DI example in the first post, stoner just moved this transfer to a different place that is in line with the barrel. In Stoner's design none of the energy is transferred to the piston, it goes to the carrier which simply drags the bolt along for the ride.

[MistWolf]

They are not the same. Do not forget for every reaction, there is an equal and opposite reaction. Draw the arrow in the opposite direction of the thrust to the carrier and what will you find? In teh Ljungman system, it's the incoming gas. In the AR, it's the piston. Another significant difference is the AR uses an expansion chamber.

What is the difference between a piston & cylinder system where the piston moves or the cylinder moves?

Stoner himself wrote that his system is not a DI system. Was he wrong?

[Treiz]

In teh Ljungman system, it's the incoming gas. In the AR, it's the piston.

... and behind the piston is more gas. The gas in both cases is trying to expand in ALL directions. As far as the transfer of force is concerned the piston is no different from the walls of the gas tube in either design, or the ground under a rocket.

[sinlessorrow]

A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings.

Sounds like a piston to me.

[Gun]

Wow, can’t believe this thread resurfaced.

In thinking if piston systems, first think of internal combustion systems. The ignition of the gun powder, like the gas-air mixture, causes an expansion. The bullet (piston in an engine), is forced to move down the bore (cylinder). There is no connecting rod to the bullet, so the bullet continues onward until over come by other forces. The expanding gases exhaust into the gas port and out of the muzzle, much the same as in an engine via the exhaust valve.

The gas tube, gas key, and gas chamber inside the bolt carrier make up the other cylinder in this system. The gases expand in this ‘cylinder’ until enough force overcomes the inertia of the BCG-buffer, and the friction created by surface contact areas felt by the BCG-buffer. The bolt tail area would be considered a stationary piston.

The conception of this being a piston system, because the bolt acts like a piston, though initially stationary, and not a direct impingement is FALSE. Expanding gases are the actuating (impinging) force in this system, NOT THE BOLT! Even in the labeled ‘Piston system’ for the AR, the expanding gases STILL provide the impinging force for the motion of the BCG.

Both systems use an expanding gas to impart (impinge) a force to move a mechanical system after the mechanical system’s inertia is overcome. Only difference in the two AR systems would be the physical size of second ‘cylinder’, where and the impingement takes place, and where the gases are exhausted.