For what its worth, just to clear up some confusion about the term "firearm", there are two different definitions; title 1 "firearm", and title 2 "firearm".
Title 1 "firearms" can be made without a license, without government notification, and without serial numbers (unless for subsequent transfer).
Title 2 "firearms" need to be registered with the government before making and include the kind of stuff one seldom sees at a typical gun shop, like machine guns, silencers, destructive devices, stocked pistols, smoothe bore pistols, sawed-off shotguns/ rifles, etc.
Then there are exemptions which turn title 2's into title 1's, such as Liberator pistols, Broomhandles and Hipower pistols with matching numbered stocks... the list goes on, but for those concerned who haven't resherched it, the quickest fix is to call BATFE in washington. I have found them to be most helpful in the past. They would much rather answer questions before the fact than conduct an expensive investigation afterwards.