Experimental American chain gun
The XM913 is an experimental American chain gun produced at Picatinny Arsenal. The cannon is a larger and more modern version of the 35 mm Bushmaster III chain gun, which itself is a larger version of the 25 mm M242 Bushmaster cannon. Although its shells, 50 x 228 mm, are twice the diameter of the 25×137mm cartridge of the M242, the 50mm cannon is not much longer than the smaller weapon. The overall lengths of the 25mm cannon and 50mm cannon are and , respectively; while the portion of the gun that intrudes into the turret are and , respectively.
The XM913 has been selected as the primary weapon on the US Army's new Next Generation Combat Vehicle, the XM30 MICV.
Ammunition.
The XM1204 High Explosive Air Bursting with Trace round is programmable. The gunner can select from three detonation modes. 
The XM1203 Armor Piercing Fin Stabilized Discarding Sabot with Trace. The projectile itself is 50 mm in diameter, encasing a long thin dart, designed to pierce armored vehicles. A light "sabot" grips the barrel's rifling, in order for the shell to get the full power of the expanding gas. Tracer rounds have a button of pyrotechnic chemical that leaves a trail to help a gunner observe the trajectory/path of the shot.
The XM1202 Target Practice-Tracer round is a full-bore training round 50 mm in diameter. With the intended purpose of drill firing and gun ballistic tests, the XM1202 round likely shares similar ballistic performance to the XM1204 HEAB-T round. Tracer rounds have a button of pyrotechnic chemical that leaves a trail to help a gunner observe the trajectory/path of the shot.
References.
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