Piston rifles

[TexasPatriot.308]

I am an old Vet (served '72-75) using the old M16, have owned and still own AR15s and AR10, AKs etc, I guess I am out of touch, but what is a "piston rifle" what makes them different.

 

[RyanM]

Many, maybe even most, gas-op rifles use a piston and operating rod. The M1 Garand, M14, FN FAL, and AK-47 are all "piston rifles." The AR-15 design, and by extension the M-16, M-4, etc., are all "direct impingement" (DI) gas-ops, meaning the powder gas goes through a tube, goes inside the bolt carrier, and pushes on it directly, rather than through a piston and rod. This has the advantage of straight-back recoil thrust, no torque on the bolt carrier so there's less friction, and no torque on the barrel either, preserving accuracy (if you watch high speed videos of the AK-47 firing, the off-center thrust from the gas system is so violent that the barrel will actually whip up and down slightly after each shot!).

These days, "piston rifle" often refers specifically to an AR-15 that's been redesigned to use a piston instead of the DI system.

The theoretical advantage is that the piston keeps most of the fouling out of the receiver. One theoretical disadvantage is that a piston is much smaller, and thus has much less bearing surface, than the bolt carrier--meaning X amount of fouling inside the piston tube will cause more friction than the same amount inside the receiver. A definite, real-world disadvantage is you also lose the straight-back force on the bolt carrier, and all the good things that brings. Many companies advertise an "anti-tilt" design, but "anti" tilt is not the same thing as "no" tilt, and the thrust is off-center in either case.

 

[DougW]

Piston rifle uses a piston to push the bolt carrier back like a FAL or similar to an AY. No gas in the chamber area. Supposed to improve reliability, run the rifle cooler. My opinion is that it is an answer to a question that isn't asked too often. I am not planning on changing from my Stoner type of AR's.

 

[LRS_Ranger]

Having a piston on an AR is a solution to a problem you don't have. While the piston DOES make the rifle more reliable in adverse conditions (look at the US army sand test with the 416 and M4) and keeps the internals of the rifle cleaner and cooler, it's not a problem that LE or civilians really have. Now, I own a piston gun. I don't own it because I think that the DI system is sub-par. I own it because I think that it is more refined, more reliable (under conditions I admit I will most likely never use my personal gun in), and I like not having a messy gun when I am done. It generally runs better with a short barrel and a can, which is what I want eventually. And lastly, I'll admit that I like having the latest and greatest... It's all personal preference, I don't see any problem with the DI, just different strokes for different folks. You own a Glock and I own a Sig. Same type thing.

 

[Quentin]

Excellent answer, RyanM!

 

[Balog]

Piston operated rifles are just by their nature cleaner and cooler than DI. However, AR's are not designed for piston op and trying to shoehorn that system into the frame can lead to issues. In my mind, one should take the AR for what it is rather than introduce a whole new and different set of issues.

 

[C-grunt]

I like others here have no problem with the DI guns.

However the piston guns do have one advantage over the DI guns. Short barrels. The SWAT team for my department recently switched from Colt ARs to POF rifles. They almost exclusively use really short, like 10 or 11 inch, guns and the Colts would sometimes malfunction. The POF rifles, according to the guys I talked to, have been running wonderful.

Plus POF is local, so there is no big turn around for parts or repairs.

 

[rcmodel]

I have never seen a published rifle test so far where any piston AR matched the accuracy of the orginial DI AR in similiar rifles.

There is also the issue of the piston guns introducing uneven force to the top of the carrier.
That results in carrier tipping, that can lead to excess upper receiver wear.
At any rate, the DI system hits it dead center behind the bolt, and the piston hits it on top about an inch higher off-center.

The piston gun has to set up undesirable barrel harmonics that would not be there with the old DI system.

rc

 

[MCMXI]

I have never seen a published rifle test so far where any piston AR matched the accuracy of the orginial DI AR in similiar rifles.

Have you read any rifle test where anyone actually compared truly similar rifles? By similar I mean same manufacturer, same barrel, same twist, etc., the only difference being that one is DGI and the other GP. For instance, how about a comparison between a Bushmaster 16M4 and a 16M4-GP? Every test I've read compares a GP from one manufacturer to a DGI from another. Regardless, I suspect that the functional accuracy differences between a DGI and GP AR are completely irrelevant if they exist at all.

The piston gun has to set up undesirable barrel harmonics that would not be there with the old DI system.

But this is true for a number of gas piston rifles that seem to be capable of excellent accuracy. The M1 Garand and the M1A for instance. Harmonics can be accounted for if you reload. If you don't, and you mostly shoot XM193 or similar, a GP or DGI isn't going to make much if any difference. My 24" DPMS DGI upper shoots in the .3s at 100 yards with my reloads, but with XM193 it's 2 MOA at 100 yards!!

The POF rifles, according to the guys I talked to, have been running wonderful.

I'm a fan of POF rifles and own two ... a P415 and a P308 ... and after the initial break-in period (NP3 coating on BCG) they've been outstanding i.e. 100% reliable. I've made some upgrades such as roller cam pins and TUBB CS flat-wire springs and I have ZERO regrets with my choice of GP ARs. POF makes some of the best lowers/uppers today. I have DI ARs too and have no problem with that system either.

 

[RyanM]

I have to wonder if the 10-11" piston guns are using an ultra-short piston. Every 10-11" DI AR I've seen has been using a carbine-length gas system, which is basically like putting a rifle gas system on a 16" barrel. That short a barrel would probably be better served with a "pistol length" gas system, or something intermediate between pistol and carbine. Just use a "dummy" front sight and use carbine length handguards, like a shorter version of a "dissipator" AR.

 

[Zerodefect]

10" barrel on an Ar is a bad idea all around. Should just switch to a different weapon system for entry, one designed specificly for that task, or stick with 14" M4's.

You can use an extra large piston to make up for the loss of pressure. Then use an adjustable port to get it just right. But I also don't like the idea of the gas port that far towards the end of the barrel.

 

[benEzra]

I have to wonder if the 10-11" piston guns are using an ultra-short piston. Every 10-11" DI AR I've seen has been using a carbine-length gas system, which is basically like putting a rifle gas system on a 16" barrel. That short a barrel would probably be better served with a "pistol length" gas system, or something intermediate between pistol and carbine. Just use a "dummy" front sight and use carbine length handguards, like a shorter version of a "dissipator" AR.

On a DI gun, the position of the gas port determines the chamber pressure at the time the bolt sees first gas; the size of the gas port determines how much gas is dumped into the system; and the amount of barrel between the port and the muzzle determines the duration of the gas dump (dwell time).

The problem with moving the gas port closer to the chamber is that the closer the gas port is to the chamber, the earlier the gun tries to unlock the bolt, the higher the pressure inside the chamber, and the harder the case is to extract. You want the gas port as far away from the chamber as you can (as long as you have enough barrel in front of it for sufficient dwell time), because it is better to unlock the bolt when the gas pressure in the chamber has dropped to (say) 3000 psi than when it is still (say) 10,000 psi. Attempting to extract too early leads to the extractor losing the case and also requires much more force, and in an extreme case the extractor can tear through the rim and leave the case still in the chamber.

I'm no expert, but it is my understanding that the carbine-length gas system (7.5" gas port position) was originally designed for 10" to 11.5" barrels and that the 14.5" barrel would optimally have the gas port at ~8.5" instead of 7.5", so the carbine length system is arguably within its operating envelope until you go shorter than 10" or so (especially if you are using a moderator, Noveske brake, or sound suppressor, which increase dwell time).