Abstract: A sub-caliber projectile cartridge for automatic weapons includes a case, a piston, a forward cavity/an inner bore in the piston, a sub-caliber projectile, and a link. The sub-caliber projectile is seated into the forward cavity/inner bore located in the forward portion of the piston. Gas passages permit propellant gases to bleed into the cavity behind the sub-caliber projectile and to accelerate the sub-caliber projectile upon firing. The case and piston are assembled and telescopically secured by means of a crimp or another similar feature, to allow relative movement therebetween during chambering and firing. The link may be allowed to move for some portion of the required relative travel between the link and the base of the case. A shoulder on the piston provides a stop through which reaction loads are transmitted to the rear of the barrel during chambering and firing.