AR's: Gas vs. Piston

[TMan]

I must admit that although I'm not a big AK fan, I've felt that the design of the AK and SKS to the AR was superior. Now we see a lot of manufacturers jumping on the bandwagon with piston operated AR's, which I've felt were superior.

Then I saw on TV that someone said that the piston operated ones weren't as accurate (sic) as the gas operated ones because the action of the piston effected the harmonics or the barrel.

Is this just someone's theory? Does it make any sense?

[MLB]

I'm far from knowledgable on the topic, but it does make sense. A piston is a moving part in close proximity to the barrel. I read an advertizement about a Savage rifle I think that was quantifying significant improvements in accuracy* resulting from thier floating barrel and method of clamping down the action.

I found an article to the point: savageaccuracy.com/2009/12/12/to-bed-a-rifle-layne-sampson/

* I'm not as pedantic about the accuracy/precision thing as I might have implied. I promise not to pounce. ;D

["DoubleAction"]

They got Layne Simpson's name wrong on the link "Sampson".

I have a floating forend on my Colt HBAR.

[TMan]

I understand the importance of bedding, and have experienced improvements by free-floating a barrel. An exception was with my buddies Ruger 10/22 International version that had the stock going way out on the barrel and then having a band at the end by the front sight. We free floated it, and the groups really opened up. Then we put the band back on and the groups tightened back up.

The thing with the piston that puzzles me is the fact that it doesn't start moving until the bullet is past the gas port. I think they have a spring that holds the piston in place, which means it wouldn't be rattling around prior to operating.

So maybe Stoner's design wasn't so bad after all.

[MLB]

It still surprises me that the method of attaching (or not) the barrel to the stock has any impact on the trajectory at all. Many folks much more knowledgeable than I think so though.

[TMan]

MLB, I've seen enough guys with tunable barrel weights that I'm a believer. The vibrations of the barrel are much like the sine-wave in AC power. Ever notice plugging something in, or unplugging it, when it draws an arc (not including motor's that is a whole different issue), it all depends on where in the sine wave the voltage is when you plug in. It can vary from 0 to 156 volts (for standard 110V circuits). Well technically from -156 to + 156, but my point is that it passes through zero.

So the barrel is oscillating as well and at some point in the oscillation the bullet leaves the barrel. If it always left at the same point in the cycle there wouldn't be an issue. Going back to the voltage analogy, if you could always plug it in when going through zero, never a spark. If we went to a 440V circuit, there would be even a larger spark if you plugged it in when the voltage was at its peak of 622V. However, if you had only a 10VAC circuit, your peak would only be 14V.

So, if you can reduce the peak amplitude of the vibrations in the barrel, which they do with barrel tuners, there is less chance of being at a peak one time, going through zero the next time, etc.

Clamping my friends barrel to the stock, the way Ruger designed it, vastly improved the groupings.

What makes no sense to me is free floating the barrel. It would seem to me that a barrel that is only held back in the receiver would have greater harmonics than one that was securely fastened to the stock throughout its length, but what do I know?
If I was smart, I'd be rich.

[5ontarget]

Some of the reasoning behind free floating is to eliminate pressures as the barrel heats up. As the barrel heats up, it can expand and in some cases warp/bend. Free floating helps eliminate some of the pressures so performance is more consistent. On some old Russian M91/30 snipers I've worked on, there was a small piece of oiled cloth near the muzzle end between the barrel and the stock. I've done some reading on this and this was fairly common practice. As the reciever was tightened down into the stock, the cloth would apply upward pressure on the barrel. My thinking was again to control how the barrel changed when heated, but perhaps more importantly to dampen the harmonics some and to try to make it more consistent as well. Having shot enough of these old Mosin's, I'm a believer, at least for these guns. I've shot the same gun with and without the oil cloth and it shoots better with the cloth. I have not done this with any other guns.
To further confuse the issue, my best shooting milsurp is my K31 and it has a free floated barrel.

back to your original question, what was that again...
Oh, yeah, I'm going to muddy the waters some again. I've read some, on the differences between short strok and long stroke piston guns. Short stroke pistons are accepted as better shooters according to much internet lore. I'll have to read up on some of this as well.

Oh yeah,
then you can also talk about how the reciver is clamped into the receiver, and how tight to make it.

[TMan]

"Short Stroke vs. Long Stroke"... well, my first wife preferred... Naw, we aren't going to go there. ;D

Hmmm, the stroke has to be long enough to move the bolt back far enough to strip the next round. Ahhh, but the piston probably doesn't go all the way back with the bolt. If it gives it enough of a whack initially, the momentum of the bolt will complete the cycle. Am I right, or totally off track?

["DoubleAction"]

Barrel Harmonics, Pressures and Timing

www.shootingsoftware.com/barrel.htm

www.varmintal.com/atune.htm

www.varmintal.com/amode.htm

www.rifle-accuracy.com/harmonics.htm

Why Floating a Rifle Barrel Increaes Accuracy

[5ontarget]

Piston/alternatives to the traditional AR gas system also has some advantages in making a pistol.


RRA's new PPS - piston system.


and the more traditional gas system.

[Callahan]

Doesn't look like my PPS.

My son is pleased with his GI Rock River AR. I've shot it and it is very accurate.

I prefer a good wood-stock deer rifle in .30-calbier or .270 to these new-fangled "black rifles" the young 'uns are all excited about!

I don't understand it. Something about zombies...