XM913 chain gun
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 The XM913 chain gun is an autocannon designed for the US military, that fires a variety of munitions with a diameter of 50mm.[1] The weapon is a scaled up version of the 25mm bushmaster autocannon, used on the Bradley fighting vehicle, a variety of armored cars.[1] A navalized version of the autocannon serves as the main armament of a variety of smaller vessels, as well as serving as a secondary armament on some larger vessels. A chain gun is the name given to autocannons that rely on an external power source to cycle munition through the weapon. This is unlike man-portable firearms, and most other autocannons, which tap some of the expanding gases of each round to power the weapon cycling through to the next round. Chain guns allow the gunner to control the rate at which the gun fires. In addition more traditional autocannons stop when they try to fire a defective "dud" round. Externally powered chain guns skip past duds, and keep firing. While the 50mm XM913 fires a larger round than its 25mm predecessor, the gun is only slightly larger, meaning that many of the exising vehicles that mounted a 25mm cannon can have it replaced with the slightly larger but much more powerful weapon.[1] The maximum range of the cannon is believed to be 7 kilometres.[1] Ammunition The autocannon fires three different kinds of color-coded ammunition.[1] Ammunition with a blue warhead are training rounds, which fire an inert warheard. A round with a black warhead is the XM1203 a kinetic energy penetrator, where a thin dense subcaliber rod is surrounded a "sabot", that fits within the cannons rifled grooves to provide spin.[1] This munition is an example of "Armor Piercing Fin Stabilized Discarding Sabot" (APFSDS). A cannon can accelerate a long thin rod, in the center of an APFSDS munition to a much greater velocity than a full size kinetic round. A round with a yellow warhead is the XM1204, a programmable high explosive munition capable of an air burst mode.[1] When a gunner selects the air burst mode of operation he first depresses the trigger halfway, which tells the aiming mechanism to determine the range to the target. Once the range is acquired he adjusts his point of aim to just above, or just beside the target, and fully depresses the trigger. Once fired the munition travels to just past the range of the target and then explodes. This mode is used when an enemy is behind a relatively inpenetrable obstacle, like a wall. The munition's shrapnel and blast can kill or wound enemies behind obstacles, without having to penetrate those obstacles. References
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