Interesting question. I don't have a good answer, you ought to ask the BATFE.
I do recall that at one time, back in the 1950s-60s, they did NOT worry about cannons (even modern breech loading ones). You could even buy a surplus tank with its cannon intact. You couldn't get the tank's machineguns without the usual NFA stuff, but the tank cannon (or artillery piece) was a no-nevermind. And that was because they registered each round of ammo (the exploding stuff) as a destructive device. You could have a cannon, and ammo with solid shot, and they didn't much care.
I'm fairly confidant that they have changed their interpretation of the rules by now, but back then, the gun itself, was no big deal.
Now, an automatic gun (AA or aircraft) was a different matter. While each round of exploding ammo (20mm and up) was a destructive device, the gun was a machine gun, IIRC.
For re-enactment, does the gun actually need to fire? I would think the best thing to do would be to find a couple barrels, and have someone make up a dummy reciever for your display. It could even be made to take the magazines and the bolt move back and forth, if you wanted, as long as it was a non-firing replica, I can't see the Feds getting involved, provided you had no live ammo.
Just my opinion, and worth what you paid for it.
Now that I think further on it, it may be that the artillery/cannons had their breechblocks pulled before being released for sale, I just don't remember for certain. If so, that would explain why the Feds didn't much worry about them.
All else being equal (and it almost never is) bigger bullets tend to work better.