With N-SSA competition you have a set of rules and procedures that require full scale cannons and mortars along with safety requirements dealing with bore liners, vents, and condition of the tube unless grandfathered in under previous rules. Also rules on the type and amount of powder used, packaging and storage of powder, the number of charges, and so on.
There are no rules and procedures by a governing body that cover subscale guns and mortars, bowling ball mortars, and the like that I am aware of. So if there is enough interest and a range or ranges available this is your chance to start a new organization that has rules and procedures for that type of competition. In an organization such as this you could make all sizes and types of artillery open for competition as long as the pieces met certain specifications for safety.