[Rick Lee]

SIG Sauer patented a special arm brace, which is basically a buttstock, but called an arm brace, for sub-16" rifles. They are so similar to regular rifle buttstocks, you would be hard pressed to explain the difference. So basically, you can do any kind of AR build you want with a sub-16" barrel, throw on an "arm" or "pistol" brace and, voila - it's a totally legal SBR, but legally called an AR pistol. I would think ATF would oppose this just on the grounds of it denying them all the revenue for SBR applications. But they have approved it and there many, many millions of them out there.