The right base and chase diameters for guns?

So I got my Smalltalk code for computing the weights of Dutch guns written and working, to a limited extent. One immediate issue that is really critical is how to choose appropriate chase and base diameters for a gun of a particular weight and shot weight. I am probably not using the correct terminology. My program needs the diameter of the gun at the muzzle end and the breech end. Both have rings that are of specific diameter. Naval guns generally flare to the muzzle, while army guns have a ring around the muzzle. The sizes determine how much metal is in the gun and there must have been design rules used by gun manufacturers that they had developed over time through experience and knowledge of what was common practice.