View Full Version : Teach me about Direct Impingement
I understand the concept that DI is the source of moving the bcg. What confuses the issue is that chamber deposits are clear evidence that the gases also exhaust from the action end of the barrel.
For them to have no effect would be from timing delayed enough to have no propulsive force against the bolt. My anecdotal impression is that blowback on the bolt still occurs and is a factor, regardless of what the action is.
It's hard to conceive there is no effective propulsion at all in DI when the same force is the actual method of cycling in other firearms. Taking advantage of it through appropriate timing would be more efficient.
A blowback or roller lock delayed firearm is cycled exclusively by propulsion from gas pressure against the bolt face. Having DI simply unlock the bolt at the appropriate time would be sufficient. Changing port sizes will certainly affect the amount of gas flow, but it also changes the timing pulse.
I appreciate the concern that DI may be depicted inaccurately, which is why I hoped someone would come in with data and time pressure graphs to illustrate. I sell auto parts and constantly deal with questions from a completely uneducated public. I'm no stranger to the concept that "Just because I say so." lacks sufficient detail to communicate information. I explain it anyway, and sometimes get something across. I also know I don't know everything, and to question common assumptions, like "that's the way it is."
Raising questions about how the DI system works has shown me it's not the jammomatic many believe it to be, including myself. While of limited value to the public, how it works exactly is beneficial knowledge, just like understanding that you can't charge a battery fully in less than an hour. More like twelve. Having the right operating concepts makes related decisions more appropriate.
Let me get this: DI has no related gas propulsion affect on the bolt face at all?
Let me get this: DI has no related gas propulsion affect on the bolt face at all?
From people much smarter than me...actually the reverse is true,
When gas reaches the expansion chamber in the BCG the pressure works in all directions within which causes the parts that can move to...well, move...
The carrier pistons to the rear with enough energy to complete it's cycle, but the gas pressure also works against the rear of the bolt.
Initially, the bolt is driven back by the pressure within the case working against the head of the case, firmly pressing the rear of the locking lugs on the bolt against the front of the lugs in the barrel extension.
When the gas enters the expansion chamber, the pressure working against the rear of the bolt literally pushes the bolt forward since the area that the gas pushes on the bolt is greater than the area that the same pressure pushes against the head of the case.
That forward movement of the bolt is critical to the longevity of the bolt and to the operation of the DI system.
The forward movement greatly relieves the shear forces working on the numerous (but relatively small) locking lugs on the bolt as the bolt turns to unlock as the carrier moves rearward. Relieving the shear forces helps the bolt locking lugs live longer since they are relatively weak to that lateral force.
It also significantly reduces the force needed to turn/unlock the bolt so the system has more energy in reserve to overcome other potential issues when cycling (residue buildup, lack of lube, dirt/grit, etc...).
MikeN
Cueball1897
05-02-10, 11:14
Here is an animation comparing the cycling of the HK416 (Piston) and the M4 (DI)
Click on "Compare to M4" to see DI.
http://www.armytimes.com/projects/flash/2007_02_20_carbine/
Not exactly what you are looking for, but visual aids are always nice.
This thread starts with a reply to an ongoing discussion, if things aren't clear, bear with.
The short version is "What propels the bolt carrier group with enough energy to cycle it? Gas from the tube or case pressure on the bolt face?"
I'm coming from the perspective the gas in the bcg is to unlock the bolt, and the case drives it back.
1) Blowback and delayed roller lock work off case pressure against the bolt. This includes most auto pistols and the HK/CETME rifles.
2) Timed unlocking is to prevent damage from an uncontrolled bolt from calibers bigger than .380/9MM. The TM calls out a serious caution that the bcg should never be assembled without a cam pin, as the results could be lethal. The bolt wouldn't lock and the carrier would be propelled backward with 50% of the energy meant for the bullet.
There is a opposite opinion that the bcg is forced back by the pressure delivered by the gas tube. I asked if there was any case pressure involved, or none.
Since two types of semi auto firearms are exclusively operated by case pressure against the bolt face, I think it entirely possible it's the same process with DI. The gas tube delivers enough to open the bolt, the case drives the bcg back.
One symptom of case pressure is that the cam pin is forced to the side of the upper channel, creating friction and wear. I believe that happens during extraction/ejection, and is the cause of the TM's instruction to generously lube the channel. On the return, only chambering the round would cause pressure, at a much lower value.
This same concept is what I believe is happening with a piston gun - it moves the bolt carrier to unlock it, the case propels it to cycle.
In the case of DI, the cam pin gets a lot of wear - and I don't think its coming from being pulled back during the cycle, but forced back. The size of the pin also seems to indicate that, too.
What's missing are some pressure graphs and data to show which contributes how much and when in the cycle, and I'm still looking.
I believe that the DI is the major cause for the bolt/carrier group moving back. I bet case pressure helps *some*, but I suspect it is only a negligible amount.
As the DI opens the bolt, it also pushes the bolt away from the bolt carrier, propelling the BCG backwards. This happens quickly. The BCG has enough momentum to keep going backwards.
I'm guessing by the time the DI has unlocked, most of the case pressure is gone.
I remember seeing a dual AR-15 full auto that had the gas tubes crossed from one upper to the next. It was made by Tromix. If case pressure was necessary to the operation of the action, this could not be possible.
http://www.tromix.com/Projects_o_Tromix.htm
Heavy Metal
05-02-10, 22:58
If case pressure was what moved the bolt carrier to the rear, gas port size would be kind of moot after a point wouldn't it?
Yet it isn't. That is a huge honking clue.
Most auto pistols are not blowback. Most are recoil operated.
With the recoil and delayed roller locking system the cycle of unlocking starts at the moment of firing. The rollers delay unlocking until the pressure has dropped and the bullet has exited the barrel. The momentum impaired to the mass of the bolt then completes the cycle and extracts the case. The key is that the momentum starts before the pressure has dropped. If the pressure has dropped then there is nothing to initiate the cycle.
On the DI system, gas from the barrel initiates the movement of the bolt carrier. The inertia of the carrier then completes the cycle after the pressure has dropped and the bullet is out of the barrel.
Aurispector
05-03-10, 19:01
Please correct me if I'm wrong, but it appears to me that both of those army times animations are incorrect as they show the bolt rotating and unlocking before the BCG moves. The animations also don't show any movement of the bolt relative to the BCG. As I understand it, the bolt lugs are locked behind the chamber lugs when the bolt is closed.
When the round is fired, the bolt can not move or rotate until the BCG moves rearward from the DI gas pulse, so the initial impulse of the cartridge base against the bolt face is effectively nullified. As the BCG moves to the rear, the cam pin rotates the bolt and the bolt unlocks. Momentum of the BCG (and possibly residual barrel pressure) then move the bolt rearward, ejecting the spent shell, then stripping a new cartridge from the magazine as it move forward again.
As the bolt moves forward the bolt lugs go past the chamber lugs, and as the BCG comes forward the cam pin rotates the bolt, locking the lugs for another cycle.
You’re completely correct, the animation is wrong and the M4 works just as you described.
rifleman2000
05-04-10, 11:10
From people much smarter than me...actually the reverse is true,
When gas reaches the expansion chamber in the BCG the pressure works in all directions within which causes the parts that can move to...well, move...
The carrier pistons to the rear with enough energy to complete it's cycle, but the gas pressure also works against the rear of the bolt.
Initially, the bolt is driven back by the pressure within the case working against the head of the case, firmly pressing the rear of the locking lugs on the bolt against the front of the lugs in the barrel extension.
When the gas enters the expansion chamber, the pressure working against the rear of the bolt literally pushes the bolt forward since the area that the gas pushes on the bolt is greater than the area that the same pressure pushes against the head of the case.
That forward movement of the bolt is critical to the longevity of the bolt and to the operation of the DI system.
The forward movement greatly relieves the shear forces working on the numerous (but relatively small) locking lugs on the bolt as the bolt turns to unlock as the carrier moves rearward. Relieving the shear forces helps the bolt locking lugs live longer since they are relatively weak to that lateral force.
It also significantly reduces the force needed to turn/unlock the bolt so the system has more energy in reserve to overcome other potential issues when cycling (residue buildup, lack of lube, dirt/grit, etc...).
MikeN
I am an expert, so I am respectfully asking: Base on the above, does a piston driven AR15 have more wear and tear on the bolt (minus the extra gas that allievates pressure on the bolt/chamber lock up)?
Heavy Metal
05-04-10, 11:30
You’re completely correct, the animation is wrong and the M4 works just as you described.
The animation also screwed-up the buffer tube depth.
I am an expert, so I am respectfully asking: Base on the above, does a piston driven AR15 have more wear and tear on the bolt (minus the extra gas that allievates pressure on the bolt/chamber lock up)?
In my understanding, yes to a degree...especially in systems that begin turning the bolt while there is still anything much more than atmospheric pressure in the barrel (e.g., barrels with a long dwell time, suppressed systems, etc....)
Also to my understanding, just increased wear is not so much the problem per se...it is much more the increased shear forces on the rear of the bolt lugs.
As explained to me, as the bolt turns while being pressed against the lugs by residual barrel pressure, the forces will be increasingly concentrated on the trailing side of the rear of the bolt lugs. Those forces can over-stress a weak and/or fatigued lug causing it to fracture and flake off a chip potentially creating a malfunction or leading to more lug failures and catastrophic bolt failure as the remaining unbroken lugs have to carry more of the load.
MikeN
Thomas M-4
05-04-10, 23:44
The animation also screwed-up the buffer tube depth.
Screwed up for sure the bolt doesn't strip the round out the chamber it jumps out the mag before the bolt even begins to move forward.
The bushmaster animation is better.
http://www.bushmaster.com/anatomy_bushmaster.asp
The gas that is sent down the gas tube into the gas key of the BCG is what sends the BCG rearward. Rather than the gas moving a piston which in turn moves the bolt to the rear (like a connecting rod on a rocker arm or piston in a car engine) the gas is directly (hence DI) forced against the BCG.
Pressure on the bolt face by the cartridge should be minimal at the time of unlocking and extraction. Like others have posted if there was significant pressure at the time of unlocking the locking lugs would get a good deal of wear.
I wish I had all the pictures from ADCO, but the details of what physics are occuring during unlocking and the transition to extracting the case are poorly understood by a lot of folks.
What makes the DI system a bit unique is that the gas being directed through the gas tube and gas carrier key enters the chromed chamber inside the bolt carrier (functions as a piston), which not only drives the bolt carrier back, but applies pressure forward on the bolt (the bolt tail is in this area, the gas rings on it provide a seal, which is why they are so important to cycling), this force being applied is important, and off-spec systems will cause the rear face of bolt lugs to wear even faster - those failures are usually a combination of this, and mostly suspect metallurgy.
The bolt face remains forward while the carrier travels back, and during this motion the cam pin functions to begin unlocking the bolt - any suggestion that case pressure is require simly isn't the case, as the amount of force required to cam the bolt out of locked position in the amount of time it does means there is enough momentum in the bolt carrier group to cycle the action.
Once the camming motion of the bolt and cam pin is completed, the cam pin reaches the end of it's helical track, and the bolt is accelerated rearwards. This quick bit of impulse is why bolts tend to break right around the cam pin hole, and why cam pins demonstrate so much wear in so many distinct areas. The channel the cam pin follows is secondary in function - the bolt will unlock through a combination of the cylindrical part of the cam pin in it's helical channel as well as the top part contacting the upper receiver.
Further evidence to me that case pressure isn't involved is that when stripped cases occur in the chamber, the BCG generally cycles and loads the next round creating double feeds.
As the bolt reaches its fully forward position, the holes (two, or three in the case of the LMT enhanced BCG) then allow excess gas pressure to be vented out via the ejection port, and as the bolt carrier group continues rearward travel, the spent case is ejected.
The extractor will rip the rim off a case stuck in the chamber. Gas pressure alone is enough to operate the action of an AR.
Cut the gas off and you have a manually operated rifle. If anything, the back thrust of the case on the bolt face serves to keep the bolt from unlocking until the carrier can perform that function.
A question- how does the gas get from the carrier key to the rings? Is the carrier key hollow all the way through? Or is it from spillage as the key separates from the gas tube? If it's spillage, how is the gas contained until it gets to the rings? Unfortunately I don't have my rifle with me to study
A question- how does the gas get from the carrier key to the rings? Is the carrier key hollow all the way through?
Yes the key is hollow all the way through.
The gas key directs the gas down into the bolt carrier (the front cylindrical hole in it is hollow, with just gas vents on the starboard side). The bolt tail with the gas rings fits into this cylindrical hole, the gas rings make a seal around the outside, so when you disassemble the bolt carrier, you'll see this as a lot of open space, but the tail of the bolt usually fills it, and it's chromed because the bolt reciprocates and rotates inside it, with the gas rings making the seal against the bolt carrier, and it's chromed because the majority of gas used to cycle the action is routed through here.
http://www.dtic.mil/ndia/2003smallarms/din.ppt
I found this on vuurwapen.blog. Microsoft offers a free viewer for ppt files.
1) There is residual pressure on the case forcing it against the bolt face. It some instances it's the only thing that extracts a case.
2) DI operates the cycle all by itself. This has been the accepted dogma long before I served, but like a lot of proprietary data, never demonstrated. Thank you TACOM.
This certainly amplifies what many of you were saying.
Robb Jensen
06-30-10, 06:56
For those of you who haven't read these two articles these are very good reads which will clarify some questions asked.
http://m4carbine.net/showthread.php?t=94
http://www.ar15barrels.com/prod/operation.shtml
A question- how does the gas get from the carrier key to the rings? Is the carrier key hollow all the way through?
http://farm5.static.flickr.com/4033/4688867069_c90b6fc231_b.jpg
Now we need the pics showing the gas rings only have .071 end clearance when in the carrier. The huge gaps we are used to seeing don't exist when the bolt rings are compressed in place.
It helps debunk the gas ring gap "danger" that doesn't exist - and why other knowledgeable shooters have insisted the AR will work just fine with one ring only.
K.L. Davis's link is very informative and dovetails well with the slides from TACOM. He acknowledges that rotation of the bolt may be lifting the extractor, so the test by TACOM to spin the bolt with added weight is significant in that it couldn't achieve lift.
Two forces act on the bolt - pressure from the case, and the DI directly on the back of the bolt. It's been said DI will actually help compensate for bolt loading by it's opposing force, allowing the bolt to turn more easily. If rotation isn't lifting the extractor, what is?
Since headspacing means there is some slight movement of the bolt, could the sudden sequential opposing pressures be causing the extractor to rock on the pivot pin, creating lift? The pivot's attached to the bolt, and the bolt does move a few thousandths.
Mount a bolt where two opposing pistons can jar it with 10k to 35k pounds and rattle it, I speculate high speed photography will show it rocking.
...Two forces act on the bolt - pressure from the case, and the DI directly on the back of the bolt. It's been said DI will actually help compensate for bolt loading by it's opposing force, allowing the bolt to turn more easily...A brass cartridge expands from the pressure of the burning gunpowder and sticks to the walls. This helps reduce backthrust against the bolt face. When the pressure drops, the brass contracts. (Steel does not expand or contract as well as brass. This is why I do not use steel cased ammo in any of my wepaons.)
Most bolts lock up with no play. Are ARs different?
Head space means there is some play.
Now Im confused and have a question. What happens on a piston gun like the 416 or ADC to the gases that go through the gas key and push forward on the bolt when there is no gas key or gases going into the bolt carrier? Dont piston guns use bolts with gas rings?
Failure2Stop
07-02-10, 05:31
Now Im confused and have a question. What happens on a piston gun like the 416 or ADC to the gases that go through the gas key and push forward on the bolt when there is no gas key or gases going into the bolt carrier? Dont piston guns use bolts with gas rings?
There is no gas transfer into the BCG, therefore significantly more force is exerted on the locking lugs and lugs in the barrel extension than with the gas system the gun is designed to work with.
ARs with shoe-horned piston systems do not require the bolt to have gas-rings, though some do to properly align the bolt within the BCG.
Deleted. See below. Site is acting strange.
Now we need the pics showing the gas rings only have .071 end clearance when in the carrier. The huge gaps we are used to seeing don't exist when the bolt rings are compressed in place.
http://farm5.static.flickr.com/4052/4689500980_9f7067223d_b.jpg
http://farm5.static.flickr.com/4023/4688867039_294d090ef5_b.jpg
http://farm5.static.flickr.com/4015/4689599072_bba0a65bf6_b.jpg
http://farm5.static.flickr.com/4071/4689599060_32513299ff_b.jpg
http://farm5.static.flickr.com/4041/4693435054_302e0c993a_b.jpg
http://farm5.static.flickr.com/4023/4692800775_de463b8032_b.jpg
Well, I obviously transposed the numbers. .017, right. Even better.
And a further question (I never thought to ask) is whether gas opens the port cover, or bolt movement:
http://www.ar15.com/forums/topic.html?b=3&f=118&t=503005
I would have thought bolt carrier movement opened the port cover, but, well, read up for yourself.
LOL……I saw that thread.
I’m going to venture to say that the ejection port cover opening is based solely off mechanical action verses gas pressure.
I do not have hard data to back this up, but I’m guessing that after the energy is expended from the gas impulse of firing the rifle, that little energy is left in the form of pressure exiting the gas vents in an amount large enough to “blow open” the ejection port door.
If this was truly the case, that gas pressure alone acted to open the ejection port door, would we not eventually see gas erosion pitting on the inner surface of the door?
The gas pressure enters the carrier key where it is dumped into a piston chamber. The opening into the carrier (piston chamber) acts as a venturi. And we know that with a venturi, an increase in velocity, equals a decrease in pressure. The pressure remaining from the gas impulse has its energy robbed in the form of work expelled to move the piston (bolt). Therefore, little pressure is remaining in the form of exhaust gasses ported overboard.
This is my take on it. If I’m wrong, I stand corrected.
Well, only the "first" shot expels pressure on the port door. After that, we leave it open for the duration of that shooting iteration. I have seen residue, especially when shooting blanks, but that's the same as suppressed, the gas trap retains more.
What's isn't known is what peak pressures do exist in the bcg. It's been suggested it's as low as 1500 psi rifle, 2500 psi carbine. Where the figures are sourced is usually never given.
Well, only the "first" shot expels pressure on the port door. After that, we leave it open for the duration of that shooting iteration. I have seen residue, especially when shooting blanks, but that's the same as suppressed, the gas trap retains more.
Yes Sir, only that first shot would be with the cover closed, and that's if one decided to have it closed to begin with.
I personally dismiss the hole theory though, about carrier exhaust port gas pressure opening the cover before mechanical action does. I would have to see some real hard evidence proving otherwise. And again, if I'm wrong, I stand corrected.
What's isn't known is what peak pressures do exist in the bcg. It's been suggested it's as low as 1500 psi rifle, 2500 psi carbine. Where the figures are sourced is usually never given.
That definitely would be interesting to find out now wouldn't it.
I personally dismiss the hole theory though, about carrier exhaust port gas pressure opening the cover.
I completely agree, what would these guys suggests performs the same function in an op-rod AR?
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