Stoner AR Operating System Technical Detail

[HansTheHobbit]

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Thanks-
F2S

What is it about the roller cam pins that don't really serve a purpose vs regular cam pins?

2 by 2...hands of blue

As far as I understand it, those rollers are supposed to prevent damage to the upper from the cam pin. I don't think they have any other purpose. And as we now know, those forged uppers last a very, very long time. So if the rollers are a liability, then it's really not worth having them.

What may be worth while, however, are the cam pins that are round instead of square. They don't have rollers; they're just round. I don't know if they would be any better than the square ones, but they might be worth looking into. They may reduce wear on the upper without sacrificing anything. I have no idea, I'm just saying that it's worth seeing where it goes. My gut tells me they're okay but probably don't offer much advantage over the square ones.

[MistWolf]

I would also be interested to hear what shuts most piston ARs down. Is it something well known, such as carrier tilt, or is something that hasn't made its way into the piston vs DI debate yet?

If people understood that there are no direct impingement ARs, that ALL ARs have a piston, we would be discussing the strengths and weaknesses of having the piston in the carrier versus having it in the gas block. Instead, we uselessly debate ad nauseam the imaginary issue of "DI vs Piston"

[HansTheHobbit]

If people understood that there are no direct impingement ARs, that ALL ARs have a piston, we would be discussing the strengths and weaknesses of having the piston in the carrier versus having it in the gas block. Instead, we uselessly debate ad nauseam the imaginary issue of "DI vs Piston"

That's a common misconception. DI is fundamentally different than piston guns, at least when it comes to the AR. The AR bolt carrier moves to the rear because of the pressure between it and the gas rings in the bolt, meaning it moves straight back instead of being pushed from above. DI BCGs essentially float, placing very little stress on the rails. That's how you can get away with having that absurdly long tail. When you put a piston into the mix, you end up tilting the carrier, which causes a binding action up front on the rails. That's why most piston carriers are supported at the tail, so that they won't tilt as much. Piston manufacturers claim to have solved the carrier tilt problem, but the jury is still out on that one. Hence the debate.

[thx997303]

I guess you won't recognize the inline piston formed by the tail of the bolt and gas rings as a piston then?

Yes they differ. They're different types of pistons. But they are not direct impingement. Look at the operation of an actual direct impingement weapon to understand the difference. The Egyptian Hakim rifle is an example of an actual direct impingement system. In it, the gas directly acts on the face of the carrier, which pushes it back. You'll notice that the operation is much more like the piston of an ak47 than the operation of an AR, just that in the Hakim, there's a gas tube rather than a piston.

To support the fact that the AR 15 is an inline, stationary piston system, here's one of Eugene Stoner's patents describing the system. https://www.google.com/patents/US2951424
Other than that, I can't disagree with what you posted.

[tvfreakarms]

As far as I understand it, those rollers are supposed to prevent damage to the upper from the cam pin. I don't think they have any other purpose. And as we now know, those forged uppers last a very, very long time. So if the rollers are a liability, then it's really not worth having them.

What may be worth while, however, are the cam pins that are round instead of square. They don't have rollers; they're just round. I don't know if they would be any better than the square ones, but they might be worth looking into. They may reduce wear on the upper without sacrificing anything. I have no idea, I'm just saying that it's worth seeing where it goes. My gut tells me they're okay but probably don't offer much advantage over the square ones.

Ah I see. Well I had never had issues with square ones. I do have the roller but I get what you t saying. Instead of having the cam head move just keep it one piece. No moving parts.

2 by 2...hands of blue

[MistWolf]

That's a common misconception. DI is fundamentally different than piston guns, at least when it comes to the AR. The AR bolt carrier moves to the rear because of the pressure between it and the gas rings in the bolt, meaning it moves straight back instead of being pushed from above. DI BCGs essentially float, placing very little stress on the rails. That's how you can get away with having that absurdly long tail. When you put a piston into the mix, you end up tilting the carrier, which causes a binding action up front on the rails. That's why most piston carriers are supported at the tail, so that they won't tilt as much. Piston manufacturers claim to have solved the carrier tilt problem, but the jury is still out on that one. Hence the debate.

This is exactly what I'm talking about. Carrier tilt has nothing to do with whether or not an AR has a piston. What causes carrier tilt is an off center thrust. Yes, early designs where the piston was moved from the carrier to the gas block caused carrier tilt from the off center thrust. But that off center thrust is due to where op rod pushes the carrier. Stoner located the piston inside the carrier where the thrust (and recoil) is applied to the center line.

The irony is, if the AR actually were a direct impingement system, the gas key would be a cup against which the gas acted and would apply off center thrust to cause carrier tilt

[HansTheHobbit]

We're getting way too deep in semantics here. The AR is a DI gun. Everyone operates from that terminology and understands it. Mostly because it's the proper terminology...

Besides, a piston is defined as a moving part. So saying that the bolt in an AR is a piston isn't really accurate, for a number of reasons. If you wanted to identify something in an AR as the piston, then I guess the bolt carrier would be the likely suspect. It's the moving part that causes the action.

Regarding the AG42 (Hakim), the gas key could easily be regarded as a piston, as it moves and causes the action on the other parts. So if you want to say that the AR isn't a "true" DI gun, then the AG42 is even further from "true" DI, whatever that is. By such a strange definition, DI couldn't even exist, as something always has to move by force of the gas, so therefore there's always going to be something that you call call a piston. Even in direct blowback, the cartridge case would then be the piston.

The only relationship between an AR and a piston gun is that they're all gas operated. Saying that an AR is not DI is...well, misguided at best. And saying that a Hakim is a true DI gun and the AR isn't, I don't understand that at all.

[MistWolf]

It's not semantics, it's facts. If it were simply a matter of semantics, all gas powered rifles would be classified as "direct impingement". Look up the technical drawing of the Lungman rifle to see how a true DI system works. Eugene Stoner himself wrote in his patent application that his gas system is not a conventional DI system, but uses an expansion chamber instead. The US Patent Office agreed. Do your research. Study the rifles and see how they differ and why. Don't just repeat what you read on the internet or hear in the gun shops, do your homework

[BGREID]

The bolt does have piston rings on it.

It's not semantics, it's facts. If it were simply a matter of semantics, all gas powered rifles would be classified as "direct impingement". Look up the technical drawing of the Lungman rifle to see how a true DI system works. Eugene Stoner himself wrote in his patent application that his gas system is not a conventional DI system, but uses an expansion chamber instead. The US Patent Office agreed. Do your research. Study the rifles and see how they differ and why. Don't just repeat what you read on the internet or hear in the gun shops, do your homework

[HansTheHobbit]

It's not semantics, it's facts. If it were simply a matter of semantics, all gas powered rifles would be classified as "direct impingement". Look up the technical drawing of the Lungman rifle to see how a true DI system works. Eugene Stoner himself wrote in his patent application that his gas system is not a conventional DI system, but uses an expansion chamber instead. The US Patent Office agreed. Do your research. Study the rifles and see how they differ and why. Don't just repeat what you read on the internet or hear in the gun shops, do your homework

I'll agree that the AR is different than other DI guns, but that's about it. There's nothing in an AR that could be called a piston. A piston is the moving part of a gas driven system that causes other parts to move. If you wanted to say that the AG42 was piston operated, then you would have a leg to stand on, as the gas key works like a very short long stroke piston. But that would be bastardizing the definition of direct impingement. And like I said before, if we wanted to go down that road then we would have to reclassify direct blowback as piston operated because the empty cartridge case acts like a piston, i.e. gas causes the case to move backwards, which causes the bolt the cycle.

Like I said, if you wanted to call anything in an AR a piston, then it would be the carrier, not the bolt, as the carrier is the moving part that is directly acted upon by the gas and causes all other actions to occur, namely unlocking the bolt, at which point it operates like a direct blowback, making the case the piston. At no time in the sequence does the bolt act like a piston. Period.

This is one of the strangest conversations I've ever had. Part of me wonders if you're just trying to derail the topic for your own entertainment. I don't know who told you that the AR isn't DI, but they were operating from a horribly flawed understanding of what constitutes a piston. Just because gas is involved doesn't mean something is piston operated, at least in the realm of firearms.

BTW, the definition of DI is "a system in which gas is directed straight to the bolt carrier without the use of a piston as an intermediary." To my knowledge, it only applies to firearms; I'm not aware of any other industry that uses the term, but I could very well be wrong about that. But the bottom line is that the AR is DI, and we're not about to change the terminology because it works, everyone understands it, and it makes good common sense.