Re: "no requirement of 'registration' for deactivated firearms"
I was thinking the same thing. My home shop includes a lathe and gun drills used for making wood winds - but using that same equipment to make a firearm wouldn't be a particularly difficult thing to do. People were doing it hundreds of years ago with much more primitive equipment.
I imagine it would be much easier than going to the trouble of trying to obtain a deactivated firearm and the needed parts, and then undoing the welding, repairs, etc.