My comment about competition was aimed
(pun) at the fact that at least some of the major organizations will allow only exact replicas with full-scale bores.
I'm not a competitor, so I can't really provide any more help there. Maybe one or more of the competition shooters can offer up some additional info on the rules of various organizations.
what will be tight enough
Windage is by definition, looseness. Tight is bad, tight is to be avoided in smoothbores. Loading and firing a smoothbore requires a certain amount of clearance between ball and bore.
Too little, and the projectile could be hard to load in a fouled bore, increasing pressures beyond safe limits. Worse yet, a ball could jam in the bore, creating a dangerous situation.
Too much, and you get excessive gas escaping around the ball, causing loss of velocity.
That's the reasoning behind the standardized formula of 39/40.
The above only applies to smoothbores. Rifled guns are a whole 'nuther animal.