556 vs. AR and gas piston AR vs. 180B

[Red$tinger.308]

I know there's a buttload of Sig 556 threads already, but none comparing the intrinsic accuracy of the AR15,gas piston AR, AR180B, and Sig 556. Which one is or should be more accurate? Looking into copping another .223.

 

[swingset]

That's a loaded question.

All things equal, the direct gas AR should be more accurate but there's soooo many variables and so many variants of AR's that it's really not ever going to be quantifiable.

The gas piston uppers I have seen range reports on seem accurate, certainly combat accurate and then some. The 180B is not known to be a tack driver, mine is around a 2 MOA gun with good loads. The 556 is totally up in the air, who knows at this point?

In terms of wringing accuracy out of a specific gun, the AR is the king not only because it's the best "platform" for accuracy, but because so many accurate and high-end components can be put on it.

 

[Red$tinger.308]

I guess I'll have to wait and see what turns up then. How are piston driven rifles any less accurate than direct gas rifles?

 

[swingset]

Direct gas has the TENDENCY to be more accurate, because of several things, mainly less mass attached to the barrel, no piston or op rod to disrupt the harmonics of the barrel or flex the action.

That said, it's all about execution. An piston driven gun can certainly be a tack driver, if it's built to be, but as I said all things equal, the direct impingement system of an AR is likely to yield better accuracy than something pushing a piston.

 

[Red$tinger.308]

Thank you sir.

 

[beerslurpy]

Direct impingement has no reciprocating parts outside the axis of the barrel. The gas directly unlocks the bolt through pneumatic pressure and the bolt is pushed straight back from the barrel.

In a gas piston system, the gas piston is driven straight back, but inertial and frictional resistance from the bolt cause it to develop force at an angle to the barrel. Also, since the bolt is usually unlocked by rotating it under the force of a cam attached to the gas piston (they are usually both parts of the bolt carrier) the bolt exerts an equal force back on the gas piston and in turn upon the gas piston cylinder and in turn upon the barrel.

So with each shot, the barrel is forced downwards and to one side, after which it gyrates briefly in the opposite direction. Not necessarily bad for single shots, but in full auto fire, each additional shot will cause a gyration to be added or subtracted to the gyrations from a certain number of previous shots. So the performance might be unpredictable- variable amounts of diagonal stringing as you fire the gun.