Minié ball

…by the elongated, or cylindroconoidal, Minié ball, as it was commonly called, with a cavity in the base that expanded on explosion of the charge to engage the rifling threads in the gun barrel. Shotgun cartridges are made of paper or plastic instead of brass.

…bullet became known as the Minié ball.

…later—and far better—addressed by the Minié ball, a projectile with a conical head and a hollow base that expanded slightly from the force of the propellant charge, thereby fitting tightly into the grooves of the rifling. Somewhat later the invention of metallic cartridges (joining explosive primer, propellant charge, and projectile…

…solution was found by Claude-Étienne Minié of France, who in 1849 developed a soft lead bullet with a cavity in its base into which a conical plug was fitted. The bullet’s diameter was small enough that it slid freely down the gun bore, and the sudden inflammation of the propellant…