National Firearms Act of 1934 set out the procedure for aquiring the transfer tax stamp from BATFE, a $200 expenditure, and forms up the wazoo, including from the Chief law Enforcement officer in your area, ie, sherriff. The wait is long, and the prices high, as the Machine Gun Ban of 1989 halted all civilian manufacture of transferrabl full auto. The older ones are all that is available, and prices go higher each day.
NFA also applies to Short Barrelled, ie <16" for rifles, or <18 inches for shotguns, and Destructive Devices, such as mortars, tank guns, and ammunition for such. All legal to aquire, IF you could find something you could afford.
There is a C&R NFA list, those that are C&R eligible but still require the BATFE tax stamp, such as the 25mm French Automatic cannon, and the German Pak40 75mm anti tank gun.....I want one.....don't have a CLUE what the heck I would do with it, but I wiant one.....