Making it a non-gun (De-milling) under federal law will require that you do things to those guns that will bring tears to a grown man's eyes. State law just requires it be "non-operable", but if vague on how far away from "non-operable" is required. This is why stripped frames are clearly non-guns under MGL, and why he MSP or Boston crime lab needs to certify a gun as operational for prosecution. The real issue is how does one do it so obviously that any LE agency would recognize it, but not destroy the gun. When you consider the high profile arrests for rocket launcher tubes labeled "INERT" and for 37mm flare guns (mistaking them for 40mm grenade launchers), I would say this is an impossible task.