My son works at a class III dealer. According to their contact at BATF: If you shoulder an AR pistol with one of those thingamajiggy braces, you have converted the firearm to an SBR by that action, and you can't convert it back. At that point, you have to get a tax stamp for it. The problem with some of those braces - I'm thinking of the Sig brace in particular - look tempting to shoulder. Conversely, you can build an SBR with a real minimalist stock, and shoot it either way - as a pistol or a rifle.ShootDontTalk wrote:This much about them is fact. The BATFE has ruled that these add-on Sig stocks are legal. But, just don't shoulder it and you're fine.
SBR AR-15's are finicky at best. The gas system is always a concern. Some ammo won't operate the gas system properly, so there is that concern.
The other issue is ammunition itself. 5.56 was designed for a 20" barrel. Half that barrel length does terrible things to the performance of the round. 1:8 or 1:7 twist is a must and then you still will need to run heavier bullet weights to get adequate performance on target. Fortunately there are some better heavy bullet rounds out now.
If you're building an SBR or pistol, I would go 6.8 SPC instead of 5.56 (sorry, opinion). This much is fact. The 6.8 was designed specifically to handle short barrels without losing so much performance. A lot of people like the 300 BLK. Based on the popularity of the round, it must be acceptable as well. Either one would make a fine SBR or pistol.
EDIT: One real negative to these is muzzle blast. There are few things on earth as loud as a 10" barreled AR-15 shooting hot 5.56mm ammo.
In addition to gas system issues, SBRs can get stupid dirty inside the receiver and BCG from incomplete powder burn. I've addressed both of those issues with a carbine-length Anderson Arms gas-piston conversion kit, on a 10.5" barrel.
I'm building mine primarily as a home defense/CQB/PDW weapon. The barrel I've selected is a 1:8 twist, and uses polygonal rifling, which will give a slight boost in velocity. That is why I'm not worried about bullet fragmentation distances, or terminal ballistics of 5.56 beyond 100 yards max. At most distances I'd want to use the SBR, I'll be inside those capabilities. I have a DMR for longer ranges, and a SCAR 17 if I need more punch if I need them. However, I am considering building a secondary upper for it in 6.8 SPC, but it won't be an SBR.
Muzzle blast won't be an issue as the SBR is suppressed.