Soviet heavy tanks. Its long-ranged 120 mm cannon was designed to hit enemy tanks at extreme distances, but it was never used in combat. Of the 300 M103s built, most went to the Marine Corps. The tank was relatively underpowered and the drive systems were fragile. The turret of the M103 was larger than that of the M48 or the M60 to make room for the huge 120 mm gun and the two loaders assigned to it, in addition to the gunner and the commander. The driver sat in the hull. The gun was capable of elevation from +15 to -8 degrees. While the US Army deactivated its heavy armor units with the reception of the new M60 series main battle tanks in 1960, the remaining M103s stayed within the US Marine Corps inventory until they began receiving the M60 series main battle tank. With the disappearance of the heavy tank from US forces came the full acceptance of the main battle tank in 1960 in the U.S. Army, and in 1973 in the U.S. Marine Corps. Vietnam War Light tanks M551 Sheridan The design of the M551 Sheridan was initiated when the replacement for the M41, the T92 Light Tank, was canceled. The need for even lighter weight than the T-92 presented the design with a difficult problem: guns capable of defeating modern tanks at reasonable ranges were so large that they demanded a large vehicle to carry them, to the point where the vehicle would be unable to fulfill its intended doctrinal use as a light tank. The use of HEAT rounds instead of conventional penetrating ammunition could address this, but HEAT rounds work better with larger calibers. Firepower on tank cannons is typically the product of caliber and muzzle velocity, so in the case of the XM551 muzzle velocity was sacrificed, producing the low-velocity but relatively large-caliber 152 mm M81. HEAT rounds fired by the M81 could defeat any contemporary tank at shorter ranges, but its low velocity made it difficult to use at longer ranges, especially against moving targets. The large low-velocity gun was also ideal for infantry support, where higher performance anti-tank guns would often fire right through soft targets and their small-caliber guns left little room for explosive filler. The M81 would thus be ideal for both direct fire support as well as short-distance anti-tank engagements. The performance of the M81 gun was deficient