I think we are confusing two different things: legal title to a firearm versus physical possession of said firearm. The federal Form 4473 relates to a transfer of physical possession, as do the NFA Forms 3, 4, 5, etc. You could "buy" a gun -- even a registered machine gun -- by paying the money and receiving a bill of sale from the seller. That would establish your legal title. No federal paperwork of any kind would be necessary -- and you could be a prohibited person, even a convicted felon -- as long as possession remained in the hands of the seller (who would henceforth become the "bailee"). Governmental regulation of guns exclusively concerns physical possession, not beneficial ownership. Al Capone could legally "own" a machine gun, as long he didn't have it under his physical control.