A submachine gun is a magazine-fed firearm, usually smaller than other automatic firearms, that fires pistol-caliber ammunition; for this reason certain submachine guns can also be referred to as "machine pistols", especially when referring to handgun-sized designs such as the Škorpion vz. 61 and Glock 18. Well-known examples are the Israeli Uzi and Heckler & Koch MP5 which use the 9×19mm Parabellum cartridge, and the American Thompson submachine gun which fires .45 ACP. Because of their small size and limited projectile penetration compared to high-power rifle rounds, submachine guns are commonly favored by military, paramilitary and police forces for close-quarters engagements such as inside buildings, in urban areas or in trench complexes.