Abstract: 1. A cartridge including a substantially rigid extractable case having at e of its ends a head formed to receive a propellant and to provide a primer support adjacent to said propellant, a projectile releasably coupled to the other end of said case, a piston engaging an inner end of said projectile and extending into said casing, said piston having a shoulder near its inner end, a base cup extending into and releasably coupled to said head, said cup enclosing the inner end of said piston with said end spaced from the bottom of said cup, and seal means between the rim of said cup and said shoulder, a second shoulder on said piston in front of the first shoulder, and a shoulder in said case for cooperation with the second mentioned shoulder on said piston in deceleration and deformation of said piston.