Originally Posted by
fusilier
My understanding is that it has to do with efficiency -- that smokeless powders are more efficient, for some reason, with smaller caliber ammo. Not to do with fouling. I could be wrong though. The Lee-Metford (the immediate predecessor of the Lee-Enfield), was originally intended to be used with smokeless powder, but the British were having developmental problems with cordite and so it was designed for a .303 black powder round. Then it turned out that the barrel was ill suited for smokeless powders. :-/
EDIT --
Blackpowder weapons may have used looser fittings to allow for fouling to build up -- but I'm not sure about that.