I agree with Mullet. Still, scale back a bit. A bulged barrel is no fun.
You should start working up some optimal loads, and that will depend on the powder or pyrodex, ff vs. fff, the barreling (twist, etc), and what type of projectile you are using. Lay a large white piece of paper or sheet on the ground, and examine it after trying out different powder loads. When you start noticing unburned grains of powder, that is unburned powder. Thus, the load is too large.
That is the simple explenation of what I am trying to get across. Tons of stuff written on all this, so do some research on optimizing loads and get to work.
If you really want to shoot cannons, you can actually get black powder cannons if you have the bucks or ability to build them
I used to do that, a lot of fun.
Dane