Info[]
Using the standardized values, a cannonball weights 32 lb (14.514 kg) and has a speed in between 1250 feet per second (381 ms), 1450 (441.96 ms) and 1700 (518,16).
Calculation[]
Simple enough.
I'll do 3 ends.
Throwing it into the KE calculator.
14.514 kg and 381 meters per second=1053430 joules
14.514 kg and 441.96 meters per second=1417500 joules
14.514 kg and 518.16 meters per second=1948430 joules
All high end Wall level
Results[]
Low-end=1.05 megajoules, 9-B
Mid-end=1.41 megajoules, 9-B
High-end=1.94 megajoules, 9-B
The correct end would end up being case by case, depending on the cannon.
Keep in mind it can also be much higher (9-A) or much lower than this, it all depends on the cannon. This is just an baseline.