In cannons the bore will be the same diameter full length. Howitzer may have a tapered chamber. Mortar may have a reduce chamber.
Bottom of bore is hemispherical if possible, at least rounded and not square cornered. Square corner can catch and hold smoldering debris and are difficult to clean out.
The vent is just a round hole slightly large that your fuse. The vent should be lined if the cannon is not of a solid material.
If you are doing a scale repro, get yourself a set of plans for the gun you are interested in from
AOP. You will never regret it,
