I haven't seen rules for cannons or stuff like that, but you could probably make your own by adapting the ballista and catapult rules on pages 151-152 of the DMG. Using the catapult as the base for the cannon, you could simply increase the damage, maybe give a bonus to the aiming, and keep the reload time the same, and you have a cannon! It's a quick fix, but if you can't find anything better this may be your best bet.