Abstract: A muzzle-expellable gun cartridge comprises a case including a side wall pted to fit the gun chamber and having an open front end and a transverse rear base, a projectile releasably mounted in the front end, a main propellant charge within the case in the front of the base, an auxiliary propellant charge located behind the base for propelling the case through the barrel after propulsion of the projectile therefrom, a primary charge for initiating the two propellant charges, and means for providing spaces between the case and the chamber and for venting part of the propellant gases to these spaces between the case and the chamber to collapse the case side wall and facilitate expulsion of the case, comprising an annular series of external longitudinal flutes and intermediate ribs formed in the side wall with aperture flaps in the flutes for venting the gases through the side wall into the flutes.