All I can offer you is my non-scientific / non-professional opinion.

The exploding gases will escape via the path of least resistance. My GUESS would be that if the "barrel" and your "loading gate" are stronger than the wading being used, than the wading would give way first and be blown out of the muzzle.

To be safe you can start with 10 grains, than 20, etc. and see how it goes on each try. There are also people who sell small scale cannon parts and kits (Dixie Gun Works, for example).

JHG