this may have been said before but you know smooth bore muskets were pretty accurate at 20 feet . im thinking at short range the rifling will take away more than it gives as a drop in velocity at the same pressure is going to happen when forward motion is converted to spin the bb or ball . more gas will pass the projectile if the amount of resistance to forward motion increases and the barrel isnt sealed against the projectile then the gas will take the path of least resistance and i am pretty sure the loss would be signifigant even for a small amount of spin. if you account for this you would need to increase your gas usage by a signifigant amount to compensate for what is likely to be a fairly small payback as the normal range of combat seems pretty close
