I agree the whole NFA registry is completely ridiculous, but I'm happy to see clarity from the ATF, even though I'm not sure they've ever actually achieved it on anything . Acting on the edge of their interpretations is a recipe for a ruined life, at the whim of some judge. If you end up on the wrong side of that you end up a felon, very likely bankrupt, and never able to own a firearm for the rest of your life. For myself, I would be immediately fired from my job just for being charged with a felony, let alone convicted. So I'm steering clear of anything even remotely close the lines, which means suck it up and just play by the rules and get the tax stamp. Honestly, I think the stubbie pistol stocks (even without the blade thing or a strap) are too close to the edge, and at the mercy of an errant court ruling.
+1. I to cannot see the appeal of an AR pistol, particularly in .223. An SBR carbine in a pistol caliber would be very cool OTOH. I shot a CAR 45 once, difficult to control the muzzle rise on full auto, but easy to keep em all in the black on a B 27 target with 3 round bursts. A hoot to shoot with OPA (other peoples' ammo!)