Sounds like a fun project. What you are going to have to do is determine the pressure built up by the explosion and then apply that as a pressure boundary condition on the inside surface of the cannon chamber. From there go through your SN curve or maybe even more of a fracture mechanics kind of study or use Simulation Mechanical's Fatigue solver. We don't have a chemical reaction simulation software which may be able to solve for this type of physics. In industry simulating just the projectile coming out of the cannon is not a trivial simulation and is best done in 2D.
Quick google search that might help... Overpressure calculation which would be based on the size of your charge behind your cannon. http://en.wikipedia.org/wiki/Overpressure
What might be interesting is to look at the size of the charge for the fixed cannon size and how how long the cannon would last for the given charge size. That would make a more interesting story... Maybe find a civil war cannon model @ Grab CAD to work from.