Lot of arm chair ATF agent/lawyers here. The point of me being interested is the fact the lower doesnt have a bore. On an AR, the lower is what is registered. From my understanding, if a lower was stamped under a certain cal it does not mean you cannot use uppers of other calibers. You guys are making an assumption based on what you are reading. I want to know from someone who knows, clear cut. What about the opposite. SBS registered shotgun, you put a sawed off rifled slug barrel on it? So your saying thats illegal unless I register it as a SBR as well? :?