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 4.6x30mm and 5.7x28mm 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 (like the ones used on the SR-2 Veresk and PP 2000); 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.