Everybody knows? Everybody knows you are quite wrong. Lead does not expand to fill rifling grooves. Bullets are made slightly larger than the bore so they engage the rifling. That was a big reason muzzleloading rifles were so slow to load. The Ferguson (sp?) and Hall rifles were attempts to overcome this by loading the ball from the breech. In cartridge guns, bullets were "patched" (jacketed) with paper or copper to get a better gas seal and reduce obduration in the barrel, particularly as velocities increased.
One exception to this is the Minnie ball, which has a thin skirt designed to flare out and engage the rifling, which is VERY shallow. In this case, the bullet and barrel are designed to work together. But the body of the bullet does not expand.
Patched round balls do not go skidding down the barrel. The patch engages the rifling from the start. The slower twist helps ensure the patching engages the rifling and does NOT skid by.
