A sabot-type cartridge has a one-piece body of thermoplastic material and a standard body configuration, such as that of a .44 Magnum, with a propellant chamber, primer, and an integral sabot of full caliber. The sabot is generally hollow, cylindrical in configuration, with radially extending ribs, separated by longitudinal grooves, the ribs supporting and centering a sub-caliber projectile, such as a .22 caliber. The annular mouth of the sabot is co-planar with the tip of the projectile to form a tapered funnel-like air inlet. A gas-actuated cutter means, in the form of a normally cupped metal member, is interposed between the projectile and the propellant chamber. There are sharp cutting edges on the multiple lobes of the member to slice the sabot free of the cartridge upon detonation of the propellant. As the sabot exits from the muzzle of the firearm, air pressure expands the sabot, in the manner of a spring collet, and releases the projectile. The invention is intended to, but not limited to, provide a family of sub-caliber cartridges, which may be loaded into, and fired from standard firearms currently manufactured by others.