Just spent the day prowling the web for information on what constitutes a NFA, title II destructive device. Just for my own gratification, not to argue the point. I agree with you guys.
Looks like I a dodged a regulatory bullet.
The 2 gauge cannon, being a muzzle loader is indeed an antique weapon. It may even have sporting use as a "puntgun" when loaded with birdshot.
The mortar was trickier to define. Turns out it was as simple as measuring the bore. My design when complete will have a 63mm bore. Since no nation ever created an anti-personnel round in that size I'm in the clear. Provided I don't ever attempt to make a unique anti-personnel round for it.
Regarding a genuine GI issue mortar, like the infamous Stokes. Yes, that would be a destructive device. So too is the old Japanese Knee-mortar of WWII, even though no ammo has been made for it since 1945.
Ironically, if a Stokes mortar is redesigned so that it can not fire the military round and is intended for use as a signal gun, pyrotechnic launcher or line thrower, it's not a DD anymore. Infact, it's not even classified as a weapon.
A similar situation exists regarding the M-203 grenade launcher that mounts under the barrel of an M-16. The 40mm launchers are DD. The 37mm are not even considered a weapon (flare launcher). However, if you buy any 37mm bean-bag rounds which are intended for anti-personnel use (albeit non-lethal) the 37mm launcher becomes classified as a DD and must be registered.
As for the French 75. It loads from the breech and once upon a time, anti-personnel rounds were made for it, so it's a DD too. Unless you redesign it to be a muzzle loader by welding the breech or install a barrel sleeve to create a unique bastard caliber of your own invention...provided the fixed ammo you make is not intended for anti-personnel use.
Oh and just for giggles, a molotov cocktail is a DD. But a flame thrower is not. In fact, a flame thrower isn't even classified as a firearm. I'm getting a headache. These laws are goofy.