Stoner AR Operating System Technical Detail

[lysander]

I'm just trying to demonstrate the absurdity of calling the bolt a piston. What I was trying to demonstrate is that the situation with the AR is no different than the way the adam's arms piston works. But you knew that already, which is why you won't just answer the freaking question.

Attachment 35570

Are you saying a part cannot have two functions?

[HansTheHobbit]

di piston.jpg
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I'm sure everyone sees my point, even if they're not willing to admit it...

If you call the bolt in an AR a piston, then you would have to call the adam's arms gas block a piston. As it relates to our discussion, the bolt in the AR serves no more of a purpose than the gas plug in the adam's arms gas block. In terms of unlocking itself, all it does is seal the carrier pocket. Put a few gas rings on the adam's arms gas block, and it's the EXACT same mechanical scenario.

[HansTheHobbit]

Are you saying a part cannot have two functions?

No. I already said it has two functions. It's a bolt, and for a brief moment it doubles as a gas plug. At no time does it behave like a piston. It looks like a piston, but it sure as hell doesn't behave like one.

[pinzgauer]

Even in the Ljungman, it's the expansion causing the rearward movement. If the gas tube wasn't inset into the key, then it wouldn't work.

I have no doubt that the Ljungman has some piston/cylinder effect. But also some direct impingement effect. But that would just mean the Ljungman is not a pure DI system, not that the AR is a DI. Doing rough math on the gas mass and velocity relative to the weight of the carrier/bolt/spring tells me Ljungman is not solely DI.

But the AR is clearly not DI at all as the gas enters at 90 degrees from the intended motion. It can only be increasing volume that creates the movement, there is no impingement effect.

Somehow this feels like the debate scene with the Sicilian in "The Princess Bride". ("You sir, have a truly dazzling intellect")

Also thread is moving so fast I missed the posts by lysander... sorry for any duplication

[HansTheHobbit]

I have no doubt that the Ljungman has some piston/cylinder effect. But also some direct impingement effect. But that would just mean the Ljungman is not a pure DI system, not that the AR is a DI. Doing rough math on the gas mass and velocity relative to the weight of the carrier/bolt/spring tells me Ljungman is not solely DI.

But the AR is clearly not DI at all as the gas enters at 90 degrees from the intended motion. It can only be increasing volume that creates the movement, there is no impingement effect.

Somehow this feels like the debate scene with the Sicilian in "The Princess Bride". ("You sir, have a truly dazzling intellect")

Also thread is moving so fast I missed the posts by lysander... sorry for any duplication

The gas tube is inset into the carrier just as much as any piston I've ever seen, relatively speaking. The Ljungman is DI. If we can't all agree on that, then this whole thing is nothing more than an exercise in the absurd. You can argue that impingement is the wrong word, but you can't sit there and tell me that the Ljungman isn't DI.

EDIT: BTW, this is exactly what I predicted at the very beginning of this whole thing. If you say that the AR isn't DI, then DI doesn't exist. This just goes to prove my point. Ten pages later here we are, now attempting to reclassify the Ljungman, which was supposed to be the "pure" DI, if everyone will remember.

[HansTheHobbit]

Going back to what I originally said, if you look hard enough you'll always find something that qualifies as a piston on any gas operated rifle. On the Ljungman it's the gas key (unless you want to call the gas tube a stationary piston, which I'm fine with. It's pointless, but it's six of one half dozen of the other). On the AR it's the carrier (unless you want to call the bolt a stationary piston. But don't tell me that the AR bolt is a piston then try to say that the gas tube on the Ljungman isn't, and same goes for the gas block on the adam's arms piston). I don't really care what terminology is used, as long as its uniform across all platforms.

That's why the conventional, universally accepted definition of DI is the only definition that makes any sense. Any rifle that pipes gas directly into the carrier is DI. THERE IS NO ESCAPING THAT.

Again, I'm willing to admit that the AR is special. I would be just fine calling an AR "inline DI" while calling the Ljungman "simple DI." Something along those lines would actually make sense. But the AR has to remain classified as DI, by the very definition of the term. I don't know why that's so hard to understand.

Oh, and I would also like to say that this whole argument should have ended immediately when it was revealed that Wikipedia was the source. Honestly, people, is that where we're getting our info from these days? Honestly...

[pinzgauer]

The gas tube is inset into the carrier just as much as any piston I've ever seen, relatively speaking. The Ljungman is DI. If we can't all agree on that, then this whole thing is nothing more than an exercise in the absurd. You can argue that impingement is the wrong word, but you can't sit there and tell me that the Ljungman isn't DI.

EDIT: BTW, this is exactly what I predicted at the very beginning of this whole thing. If you say that the AR isn't DI, then DI doesn't exist. This just goes to prove my point. Ten pages later here we are, now attempting to reclassify the Ljungman, which was supposed to be the "pure" DI, if everyone will remember.

Here's the difference... I can prove the AR is not impingement as the gas force is 90 degrees from motion. This is not me, this is physics, NASA and others. I can give you the formula for force created by impingement, it's not that hard. The cosine of 90 degrees is zero... it's that simple even without all the other variables.

So for me this is not an emotional debate over words... it's simply physics and engineering terminology.

As to the Ljungman... it clearly has some impingement effect, but also likely has some captive expansion effect. The designers probably had a reason to call it what it is. The question would be how much the bolt would move if the cup was larger, negating any expansion effect to allow only pure impingement. Or drilling holes in the cup to relieve gas pressure. That's the only way that would tell.

So I'm not redefining DI to prevent the AR from being called that... just conceding that there is likely some aspect of expansion in the Ljungman, so maybe it's operating approach was misnamed. Or was a compromise.

[lysander]

No. I already said it has two functions. It's a bolt, and for a brief moment it doubles as a gas plug. At no time does it behave like a piston. It looks like a piston, but it sure as hell doesn't behave like one.

It behaves exactly like an AR180's piston.

It stays stationary while a closed off round thing slides away from it.

or this piston (if you hold the shaft still and let the black part move):

[thx997303]

Why don't you understand the differences between a piston and a plug Hans? A plug functions only to seal, a piston seals and provides energy to cause movement.

There will be no convincing you if you won't educate yourself on physics.

[pinzgauer]

That's why the conventional, universally accepted definition of DI is the only definition that makes any sense. Any rifle that pipes gas directly into the carrier is DI. THERE IS NO ESCAPING THAT.

We'll have to agree to disagree... My definition of a DI system has nothing to do with plumbing, it's all about the impingement:

A Direct Impingement operating system would be one where the following conditions are present:
1) The majority of the force used to operate the system was due to the jet of gas in line with the desired operating motion impacting a surface
2) Any force generated due to expansive or increasing volume of gas is secondary (smaller) than the impingement force
3) The interface between the gas tube and impingement surface is not capable of sustaining pressure and is essentially an open interface not dependent on rings or small tolerances

But that's just based on my understanding of the physics of impingement.

We have many forms of inexact terminology... blowback operation comes to mind. There is a defacto definition of a blowback system, but the naming is inaccurate as it is not residual gas expansion blowback that creates the force... it's the "equal and opposite" force of the gas expansion pushing the bullet which also pushes against the base of the cartridge case and bolt/slide face. But the bolt typically does not move until after the bullet has left the barrel and the system is now open. So although expanding gas created the initial force, it's not a captive expansion of gas like a piston system.

But that's a different debate. If we can use the term blowback we can probably also use the term DI for Ljungman/Hakims