What case said, you need a nfa registered one. It’s not like a ar where you can get a nfa transferable registered sear or nfa transferable receiver and use a civilian aug. if you have a ffl and looking for a post sample get you documentation together and contact them directly. If your not a ffl you can look for a transferable pre may 86 one and go through the nfa process with a class3 weapons dealer, but I haven’t seen a transferable one, only post samples. if you are new to the gun world and do not understand class 3 or nfa I would highly suggest looking up class 3 and NFA or speaking to a ffl about the process. I would not mess with your gun in anyway, trying to alter it to fit a different pack will get you one heck of a felony.