It's hard to find a hard and fast rule about ownership of a tank with its main gun still intact; everything I can find online states it must have the firing mechanism deactivated.
The smaller machine guns are obviously another story, as there are specific laws about their acquisition and registration... the guys at Red Jacket obviously have all their federal ducks in a row because they play with all kinds of full auto weapons.
Whether you take the gun as an integral component or as a separate item, it is legally a destructive device.
File your ATF Form 4, pay your $200, wait 27 years for BATFE to process it and go blow something up...
If you happen to have HE rounds, each of those would also be a DD with it's own tax stamp and transfer paperwork. AP wouldn't have that issue...
Edited by nova, 14 August 2014 - 12:25 PM.