The M242 Bushmaster is a 25 mm (25x137mm) chain-fed autocannon. It is used extensively by the US armed forces, as well as by NATO's and some other nations' forces in ground vehicles and watercraft. It is an externally powered, chain driven, single-barrel weapon which may be fired in semi-automatic or automatic modes. It is fed by a metallic link belt and has dual-feed capability. The term "chain gun" derives from the use of a roller chain that drives the bolt back and forth.