Very nice. The proof load used for your little cannon appears to be the amount of powder that occupies the bore to a depth of 2.5 inches from the bottom of the chamber. For instance, IF the cannon was .75" bore, then it is pi x 0.375 x 0.375 x h - h being 2.5.
I'll let YOU work it out.
Oh, alright. It's 0.9352 cubic inches. Easiest if you make a model of the bore and fill it up to the 2.5" line THEN measure that weight of mini-cannon grade BP - say FFg for the one that you have. OR, just do the same for Pyrodex RS - NOT P!!!! The Pyrodex RS should weigh around about 90% of the equal volume of BP - remember, it is VOLUME that you are looking for.
Remember also that this was the proof load and reduce your final load accordingly - 75% of whatever it was might be fine.
tac
PS - I'm VERY interested to know what grade steel you made it from - please PM me if you care to.