AR- Piston or Direct Impingement?

[Failure2Stop]

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.

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[sinlessorrow]

The system most certainly exerts force forward on the bolt.

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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]

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]

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.

[Koshinn]

In the context of ARs, sans raw material and machinery, nothing.

Have you built a wall or fence? Or is that just assembling too, since you can get fence posts, bricks, and other materials?

[Failure2Stop]

When research on firearms leads one to argue semantics about naming conventions, one should really ask one's self, "self, would John Wayne and Clint Eastwood be having this argument?"


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[Bro KV]

Great information!!!

[Todd00000]

When this question comes up all I do now is quote Larry Vickers.

https://www.m4carbine.net/showthread.php?t=107613

Not sure if I posted this in my SME forum before but here goes; many of my students have heard this and I mention it on my Daniel Defense carbine DVD's-

Piston AR's have their place- to determine if you need one ask yourself four questions;

1) Do I need a barrel length shorter than 14.5 inches
2) Do I need to run my gun suppressed a lot
3) Do I need to shoot a lot of full auto
4) Do I need to shoot a wide variety of ammo

Piston guns have been shown to be superior in those four categories over a DI gun. If you can say no to all four questions a good DI gun will do you fine

My favorite DI brands are Daniel Defense ( who I am a consultant for), BCM ( who is one of my show sponsors along with DD) , and Colt ( who I have no affiliation with- other than a buddy who works there)- from what I have seen all three of those brands build good solid AR style rifles that are proven performers

In piston AR's the gold standard of course is the HK 416- the civilian legal version called the MR556 is basically the same gun with some changes like a heavier, non chrome lined bore that make it different enough that some people prefer to source a grey market HK 416 upper or choose a different piston gun. The MR556 has an excellent reputation for accuracy and although heavy is a very well made rifle. Another piston brand I have seen that I have seen good results with is LWRC - the majority of LWRC guns I have seen in my classes perform well; If I wanted a piston AR and did not want an HK product I would look at an LWRC offering.

The Internet, and M4C is no exception, has been full of non objective comments and threads regarding this subject- I would urge potential buyers to sort thru the emotion to determine what makes sense for them - remember any of the guns I listed with good ammo ( getting harder to find these days at a reasonable cost), good magazines and well lubed have proven to be reliable performers; in fact we live in the golden era of AR style black rifles- never before has there been so many good choices in this category

There are other quality brands on the market to be sure- I just listed the ones that I have the most experience with and have seen the fewest problems with in my classes

Now the task is up to you to take the emotion out of your decision making process and buy a carbine that you decide will get the job done

Be safe

LAV