not a flintlock (or fowler) but i have done a lot of smoothbore shooting at 25 and 50 yds in NSSA. further distances than that, all bets are off. a reasonably well-fitted ball will shoot tight groups at 50 yds, provided you get the powder charge right. i know people using a properly set up weapon have gotten good smoothbore accuracy out to 75 and 100 yds, but in my experience 50 was the last bastion of repeatability.
the biggest accuracy difference to me was realized by adding a rear sight. without it you have to use other visual cues to get a repeatable sight picture. i dont think any of the really serious shooters were not using a rear sight.
definitely a more labor intensive operation than shooting a rifled musket, partly because we couldnt use patches. so fouling and projectile fit was always a concern. ever hand-filed roundballs to make them round, then find a way to get lube to stick to them?? good times.
