the M68A1 gun was viewed as an economical and practical solution that allowed for commonality in ammunition among the two types of tanks. Secondly was the fact that the M68A1E2 could employ the newly developed M900A1 APFSDS depleted uranium round that had improved penetration performance in comparison to the M833. These early versions of the M1 Abrams were in active Army service until 1991 and with National Guard units until 1996. M1s built after 1984 were armed with the 120mm M256 and designated the M1A1. Many earlier M1 and IPM1 tanks were refitted with the M256 and their designations changed to the M1A1. The M1128 Stryker Mobile Gun System (MGS) carries the M68A1E4 105mm cannon. The principal function of the MGS is to provide rapid direct fire to support assaulting infantry. The cannon is mounted in a low-profile, fully stabilized turret integrated into the Stryker chassis. The M68A1E4 is based on the M68A1E2 105mm cannon's design. It has a sustained fire rate of six rounds per minute. The gun employs four types of cartridges. The M900A1 kinetic energy penetrator to destroy armored vehicles; the M456A2 high explosive anti-tank round to destroy thin-skinned vehicles and provide anti-personnel fragmentation; the M393A3 high explosive plastic round to destroy bunkers, machine gun and sniper positions, and breach openings in walls for infantry to access; and M1040 canister shot for use against dismounted infantry in the open. The M68A1E4 will be completely withdrawn from US service when the M1128 Mobile Gun System is retired by the end of 2022. Other users The gun was subsequently adopted for use on the German Leopard 1 (for which the L7A3 variant was developed). In addition, several countries have used the gun to improve the firepower of existing main battle tanks. Derivatives have even been mounted in Warsaw Pact-built T-54 and T-55 tanks in Israel, India, Egypt and Iraq, and Type 79 tanks in China. Variants UK models L7A1 Standard UK production variant. L7A2 L7A1 fitted with a thermal sleeve. L7A3 Variant for the (West) German Leopard 1 MBT. The upper rear corner of the breech block reduced in size so gun can be depressed without hitting the turret roof. L7A4 L7A3 fitted with an Ernest Leitz Canada muzzle reference sensor. Used by the Belgian Leopard 1A5 (BE) and Canadian Leopard C1. L7 LRF Low Recoil Force (LRF) version of the L7 used by the Stingray light tank. Royal