You can legally buy and own a barrel blank (rifled tube) provided that it is not chambered to take a round, or is not closed at one end with a touch hole. This from my FEO when we were discussing the 50 cal cannon I'm making. By this logic as soon as you take the old barrel off it will cease to be a firearm part, and the new barrle will not become a firearm part until fitted to the frame.
Discuss!!!