Based on my very unscientific experiments as a kid I would guess a bag of gun powder to be rather ineffective. We did all sorts with the gun powder from fieworks and Rook scarers (big bangers on a rope) and unless the powder was compressed and in a tight wrap eg paper (firework) it just fizzed,made a lot of smoke and some pretty lights...depending on the fireworks used,and not a lot else.So I cant see a bag blowing up a cannon....on its own anyway.