My limited experience with cannons, swivel guns, etc, is that mostly are used for blank firing, no projectiles.
So the bigger the bore the more powder you need to get a good report. This of course makes it more dangerous. I find 1" or smaller and you can get a bigger bang for your buck.
I have a 1" bore swivel gun, WITH A LINER. About 120 to 150 Gr of what ever, 1F 2F or even three, usually just the sweepings, and I get a nice boom.
My cousin built one with 2" bore, takes too much powder to be loud and we're always afraid something is gonna bust!