First, Suggest you see if an actual bowling ball will go in tank. There have been problems in this area.
Next figure out what pre 1899 mortar you are going to replicate.
Figure out what you are going to do about trunnion and trunnion mounting. If you going to make a mortar AAA guidlines says the barrel length shall not exceed 2 times the projectile length, while projectile length shall not exceed 3 times the bore diameter.
You need to evaluate what you want from this Mortar. Where are you going to shoot it? How far can you safely shoot? How are you going to move it?
Look over the posts on this board about bowling ball mortars and determine how big your powder chamber needs to be to fire your bowling ball with in your parameters.
You may need to do some reverse engineering to figure out the size of your chamber. In large bore guns the maximum safe load is computed as 2 ozs of FG per inch of bore. If a bowling ball mortar throws a ball 600 yards with 3.5 oz, the safe powder chamber diameter for that charge if it were a maximum would be 1.75 inches. (3.5/2=1.75 inches). You would have to determine how deep to drill the hole for the chamber to hold that much powder.
The powder chamber should meet all the standard safety rules as established by N-SSA and AAA. Wall thickness should be one caliber. If you are going to use the existing neck as the basis for the powder chamber take the out side dimension of the neck and divide that by three. That will give you the diameter of the powder chamber. Do your math and see if a powder chamber of that size is large enough to achieve you goals.
