Constructive possession = The sum of the parts equals the whole.
Sure, you can have a rebuilt kit with a follower, a plate, and a spring. Just like you can have a AR parts it including a trigger, hammer, and springs.
But the magazine body cannot be "rebuilt" with another magazine body.
Nor can your AR-15 Barrel be "replaced" with a 8" SBR barrel you have laying around (no tax stamp).
Just as if you have an AR-15 Rifle lower, and a 8"SBR barrel, disassembled, with no tax stamp - surprise, constructive possession means you still have an illegal SBR.
Same appies to magazines. You have a post July magazine body? Or your buddy gave you a magazine body after July 1? Well, you can't rebuild that, and the sum of it's parts still equal a whole magazine.
I will NOT go into discussion as to the probability of prosecution. The simple answer is we don't condone any illegal activity. I don't care if something would, may be, or never would be prosecuted. We're not going to support those discussions here, nor those type of transactions.
^^^^ This needs to be a large red lettered stickie at the top of every subforum
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