Projectiles for firearms, specifically bullets, are discussed that include a rear thin-walled counter bore. The counter bore is designed to be a first size and shape when the bullet is loaded into the firearm and expand upon discharge of the firearm so as to force the walls of the counter bore into barrel rifling. The expansion may occur through the direct interaction of propellant gases with the counter bore walls, or at least partially indirectly though the inclusion of an expansion plug that is placed at least partially within the counter bore in a manner that the expansion plug can be driven further into the counter bore by the firing action of the firearm.