DD, per that letter it makes it sound like the other cannons would be destructive devices, as they are intended to expel a projectile by means of explosives, and have a bore greater than 1.5 inch's...
Do you have a link to where that has been discusses so I may read up on the opposite ruling?
Your assumption is wrong and is a perfect example why Legal opinions and discussion of the law are not allowed on this board.
Pull out your copy of the GCA 68 and NFA and look up antiques. GCA for guns under .50 inches and NFA for over .50. Very simplified and generalized if it is original or a replica and it was made in or before 1898 or made before 1899 and is a muzzle loader, uses an antique ignition system and or is a breech loader using fixed ammunition that is not available in commerce it may be considered an antique and not a firearm.
That is a very none legal explanation which is not binding in any court. If you must have a proper legal definition then write a letter to ATF and ask.
Anything anyone else may pos,t and including what I just posted my be dead wrong....get the right answer from ATF.
And one other thing that I fail to mention all to often when the subject comes. Even though you get a letter form ATF, you still need to check with your State and local jurisdictions to see if the have additional or restrictive laws.