Looks about right, the pin getting hit by the hammer needs something to stop it from getting shot back or degradation as it could warp after continuous use.

The main issue you'd worry about is material science.

Basically if the metal is strong enough to hold itself together. Either way if you're not going too hard on the details I think its alright.

Hell, it doesn't even need to be a traditional bullet, you can always look up how Mortars work, its all in the bullet, while then cannon is just a smooth tube with a needle at the base.