A personal defense weapon (PDW) is a type of semi-automatic or fully-automatic firearm similar in most respects to a submachine gun. They typically have better range and accuracy than normal submachine guns. Cartridges such as the HK 4.6×30mm and 5.7×28mm use specially constructed and shaped bullets, along with high velocity to provide better armor penetration than traditional 9mm Parabellum or .45 ACP rounds.
A PDW is an SMG-sized weapon chambered in an often-proprietary round; generally, these cartridges resemble miniature rifle cartridges and exhibit similar ballistic effects, though on a smaller scale; muzzle velocities are often still not up to where a rifle would be, but beyond the capabilities of a normal SMG.
There are PDWs which use pistol-caliber rounds; some use conventional pistol-caliber ammunition and are simply more compact variants of another SMG; some use overpressured pistol cartridges; others use rifle-caliber rounds and are referred to as PDWs by the manufacturer. As with the pistol-caliber PDWs, these are more compact variants of rifles or carbines.