Per Uncle Norby: "I don't beleive it is the sloppy fit that allows them to gain more velocity. If anything, a sloppier fit would result in less pressure generated, and less velocity. BUT other factors are also at work. Because you did get higher velocity I'm sure the heavier bullets are upsetting properly and sealing the bore, allowing perhaps even a more efficient burn of the charge resulting in higher velocity. "

Sorry! I forgot to address this. I believe Uncle Norby is correct-the heavier bullets make more efficient use of the powder charge, both because they seal the bore keeping the gas behind the bullet pushing, and have greater inertia, allowing pressures to build higher before the bullet begins accelerating.

A patched round ball is the least effective bore seal of all possible ML bullets. It is so poor, PRB's didn't really work at all with GOEX ClearShot powder, giving very low MV's with that weak powder. ClearShot worked better behind conicals......