I remember seeing something on Calguns recently that said ATF is fine with it. You can take something that was originally a pistol and convert it into a rifle no problem. You can even convert it back into a pistol no problem (and back to rifle, etc). However, a rifle converted into a pistol becomes a SBR.This is how to do it. Mech-Tech carbine conversion unit.
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I don't know what the ATF thinks about putting a shoulder stock on a pistol reciever even though it meets the NFA barrel length and overall length requirements. Maybe it magically becomes a rifle?

