and in either case, all of the remaining serviceable 152 mm shells still remained caseless, albeit attached to their warheads, and sleeved into a re-usable white nine-ply nylon bag, which was form-fitted to hold the propellant portion of the shell. The white/silver-colored bag had a strap attached to the bottom, which the loader would grab and pull off prior to gently inserting the shell into the breech. Once a mine or RPG-type weapon created the spark, smoke and fire became imminent, and it became a matter of Standing Operating Procedure to abandon the tank immediately. On 15 February 1969, just one month after the Sheridan's arrival in South Vietnam, an M551 from the 3/4th Cavalry detonated a 25-pound pressure-triggered land mine, which ruptured its hull and ignited the 152 mm shells, resulting in a secondary explosion that destroyed the tank. In late 1969, nine Sheridans from the 4th Squadron, 12th Cavalry Regiment were fording a river near the DMZ, when three of the M551s detonated mines, completely destroying them. In March 1971, five Sheridans from the 11th ACR were lost in one day to RPG fire; all five vehicles burst into flames and were totally destroyed. It became a common scene to observe melted Sheridan hulls with their sunken steel turrets sitting at odd angles with their gun tubes pointing towards the sky in various parts of the country, either awaiting final disposition, or simply forgotten. Performance The Sheridan had several advantages: it did not get stuck in the mud as often as the 52-ton M48 did, nor did it throw its track off as often. This alone was enough to win the tank crews' favor. The light weight and high mobility proved their worth, and the gun proved an effective anti-personnel weapon when used with either the M657 HE shell or the M625 canister round, which used thousands of flechettes as projectiles. The reliability of the engine system and gun of the tank was, however, not fully up to the task. Of 74 M551 sent to Vietnam in February 1969, by May there were recorded 16 serious mechanical faults, 41 failed shots, 140 defective ammunitions and 25 burned engines; the turret itself had 125 electric faults, several recoil system faults and instances of blown up guns. An additional problem was that much of the carried machine-gun ammunition had to be stored outside the turret as the internal space was