I got a phonecall which interrupted typing on my reply, so now all I can do is agree with Relic Nut and Jewelerguy -- and add some information.
When a blackpowder gun gets fired all day in battle, a thick coat of powder ash and bullet-lube grease builds up on the inside of the gunbarrel. That is called "blackpowder fouling." A gunbarrel can get so fouled, you've got to use a lot of force to get a bullet to go into it. There are reports of civil war soldiers in all-day battles having to use a rock to pound the ramrod down the barrel. When that happens, the ramrod's mouth, which is shaped similar to a bugle's mouth, can make a deep impression of its lip and hollow "throat" on the soft lead bullet's nose. Relic-diggers call such bullets a "hard-rammed" bullet.
I should mention, it only happens to muzzleloader bullets and blackpowder revolver bullets.