Actually your Cannon has thin walls and wouldn't be allowed under N-SSA and AAA rules. Wall thickness is minumum 1 calibar for safety purposes. Unless you gun was an exact replica of some Civil war Cannon you couldn't shoot in N-SSA any way. So it realy doesn't matter. The AAA boys would let you play with ascale modle gun. But me my self and I, I don't think I'd worry about it. If it's 4140, I'd just shoot it!
If you got a good boom with 400 grs. Pyrodex then you have the right combination. Be carefull where you point the thing. Remember the wad is a projectile.
Black powder is preferred. There are those who say they have been able to make Pyrodex work in cannons, but I haven't. Noise yes, launch projectiles, no. I am also a bit nervous about pressure issues with Pyrodex as it generally takes a bit of resistance to make Pyrodex shoot a projectile.
Welcome to the board, by the way always glad to have some fresh faces...always seems to be a few who show up this time of year...some drift a way others just hang on...no experts here, just a bunch of fellows sharing a combined knowledge...an all of us learning.