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I have an mpx with a kak shockwave on it, but it's a form 1 sbr so I can shoulder it and use it as a stock. But if I want to take it across state lines with no additional forms, I can just say it's a pistol.
I've heard that if a form 1 lower is on a regular rifle length upper then it's no longer a nfa item and not regulated. So it should apply the same to pistols and pistol braces.
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I do not believe that it does apply the same way. It is my understanding that an SBR lower is an SBR for as long as you retain the parts to make it one. TECHNICALLY, an SBR with a KAK brace on it is still a Short Barrelled Rifle and as such cannot legally be transported across State lines without submitting the form in Advance.
I have a separate lower receiver, built as a pistol, with a Shockwave on it. If I go to F-I-Laws house one state over to shoot, I remove my short Barrelled upper from my SBR lower and put it on the pistol lower. The SBR lower stays home.
With the current going rate of a stripped lower receiver, it was a no-brainer for me. I thought about saying it was just a pistol lower too, but the SBR engraving on the right side says otherwise.
Then again, I ain't no lawyer, and I also don't recall the MPX being as cooperative with parts swapping.