By definition a destructive device (cannon) is a NFA firearm.
If it will fire more than one round per pull of the trigger, it is a machine gun. If it has a bore bigger than .5 inch and no "sporting purpose" it is a destructive device. I don't think that guns get put into more than one category. I base this on an ad for a 30mm AT gun that was registered as a post-may machine gun. The seller stated that he could convert the weapon to single fire and sell it as a Destructive Device.
All that said, If you want to start restoring old cannons, good for you. There is to much history going to the scrap yard these days. For WWII reenactments a de-milled weapon should work fine. If you want one that fires, now you are talking about some very tall cash.
edit: I don't think the ATF is allowing working anti-air cannons (destructive devices) to be imported without being de-milled first.
"No, it's not a silencer. This little doodad is my own invention. I call it a loudener." --Sledge Hammer
Last edited by Willie Lowman; March 16, 2011 at 11:06 AM.