With low explosives like those used in cannons figuring out the force can be pretty tricky. This isn't an adiabatic expansion, the force experienced on the ball will depend on how fast the ball is moving. The rate at which the explosive burns depends on the heat inside the chamber which depends on the pressure. The burn rate and rate at which the cannonball leaves, plus a modifier based on how much escapes through the ignition hole, governs the pressure. I really wouldn't trust any calculation of all of the above without experiment.