I don't know. The ATF confuses the hell out of me most of the time. One moment it seems like they are getting super lax on their determinations and they are allowing the slidefire, sigbrace, 26" PGO shotguns, and even go as far as to say shouldering the sig brace is legal. Then the next minute they are ruling a muzzle brake a silencer and telling a manufacture they can't make it.
Hell, I still don't understand how the KX3 isn't considered a silencer.
The original 308 design was classified as one.