Hmmmm.....About two months ago I was at a firearms instructor conference and I sat through a BATF lecture, given By a BATF Agent/Lawyer from their legal division, on dealing with SBRs, SBSs, AOWs, Firearms (shockwaves et al) and Braced Pistols vs their their illegal counterparts the saw-offed rifles/shotguns . He very clearly stated the BATF looks at the intended design of the brace to determine legality, not how it is used by the end user and the BATF doesn't considering shouldering a braced pistol illegal under any circumstance.
If that's true, then why even retain the classification of SBR? If you can buy or build an AR15 with a 10" barrel, and all that you have to do to make it "legal" is toss on an SB3 instead of a normal M4 buttstock... Why even do that?
Two identical guns, one is legal, one is not, and the only difference is what brand piece of plastic you attack to the end?