"So I was wondering, if you own an AR pistol with a short barreled upper, and you own an AR lower for a rifle, would the ATF have issue with that and claim you could make an unregistered SBR by putting the pistol upper on the rifle lower?"
Only if your parts cannot be configured into a legal format. If you only have a rifle lower and a pistol upper, the ATF could argue successfully that you intended to mate the two and form an unregistered SBR. If you have a pistol and rifle upper, a stock, and a pistol lower, then they must assume law-abiding intent up until you actually make the illegal configuration (assumption of law-abiding intent when both legal/illegal actions are possible; there's a latin term for it I forget)
"The gas operated AR is not the same as dealing with a blowback or recoil operated system, which are more dependent on bolt/slide mass and recoil spring rates to cycle and feed properly. The smoothest cycling results from proper dwell time and port size combined with appropriate buffer weight, but it is easy to alter gas port size to compensate for shorter or longer dwell times, and play with bolt carrier/buffer weight to soften recoil impulse and slow down/speed up cycling."
A friend is actually working on a 3-or-so inch AR pistol, that I can only guess uses some sort of gas-trap to deliver the needed back pressure from the muzzle itself. I understand it's glorious on the range, but doesn't cycle just yet.
TCB
