The military's of the world found by swaging or casting the ball closer to the bore size (reducing the windage), although increasing loading problems due to the buildup of fouling with fewer shots, it also increased the musket's accuracy. Thus, they found, as we know today, a tighter fitting ball increases smoothbore accuracy.
Attempting to somehow improve smoothbore accuracy by causing a spin is easy - use VERY coarse emery or flint paper and scratch a spin into your bore- right or left twist, doesn't matter. You can use another rifled barrel blank as a 'guide' - easy to do.
This method actually works fairly well for a number of shots before needing to be reapplied - Dan has mentioned this method here at ALR as possibly being used to day, and definitely was used many years ago.
This method might be construed as a move outside the flavour of good sportsmanship (by mechanical advantage, seeking unfair advantage in competition) and thus would/should remove that smoothbore from a smoothbore event, I'd expect.
If improved accuracy was desired from your favouite stock design, by all means - put a rifle barrel on that gun, saving the smooth one for shot.imho
Or - you might use a centre-punch and "punch" a DIVOT staight down from 12 o'clock into the bore at the muzzle or very near the muzzle. This "might" impart a backwards spin to the ball - however slight, which might cause it be more stable than normal and not take on the normal 'knuckle-ball' effect which badly effects accuracy, sooner or later depending on the velocity. If it worked, would it be a 'cheat' in competition? It just might improve a smoothbores accuracy for hunting, as well as having no adverse effect on shot patterns.
