I dont know how it is in the US, but where I live I think a suitable barrel and maybe some parts of the firing mechanism is already classified as a gun (and requires a permit to have or sell, with all the corresponding paperwork).
That way its impossible to make a gun by buying unregistred parts, and those parts that require registration are usually the most difficult to make.
__________________
Z4 3.0i | ESS TS2+ supercharger | Quaife ATB LSD | Brembo/BMW performance BBK front/rear | Schrick FI cams | Schmiedmann headers+cats | Powerflex/strongflex PU bushings | Vibra-technics engine mounts | H&R anti rollbars | KW V3 coilovers/KW camber plates | Sachs race engineering clutch | tons of custom sh#t
|