extremely limited. Although an average M48 crew could fire as many as seventeen 90 mm shells during a "mad minute" (60 seconds with all guns firing-on command), the Sheridan was known to put out only two 152 mm shells during the same time frame. While the M48's 90 mm cannon fired fixed metallic cased rounds, the 152 mm was caseless. The caseless rounds needed air vents to clear the gun tube and breech prior to loading another round, while the M48 breech block opened as the used shell was ejected and closed as the new shell was shoved in. The faster the loader, the faster the Patton's gun could be fired. For the Sheridan, the loader had to wait for the mechanism. After firing, the loader would have to wait, as the breech slowly opened rearward then turned downward. After another instrument indicated that all turret systems were still operational, the loader would gently push the 152 mm fixed round into the breech and watch the breech block slowly rotate upward, then forward into the breech, then again, wait for the lights. Over 200 Sheridans were shipped to Vietnam. The Sheridan was much appreciated by the infantry, who were desperate for direct-fire support, and generally served in armored cavalry units along with ACAVs (M113s). Armor units consisted solely of tanks (minus headquarters company) and mechanized infantry units consisted solely of M113s. In this role, the real problem with the Sheridan was its limited ammunition load; normally, only 20 rounds and 8 missiles; although, as the M551s in Vietnam service were not equipped with missiles or their guidance equipment, this increased the basic load of conventional rounds. Sheridan losses were heavy during normal operations, largely due to land mines and anti-armor weapons, but were especially heavy after the start of the Cambodian Campaign in May 1970 in which, among other cavalry squadrons, the 11th ACR was thrown into the fight. The second heaviest losses were during the U.S. Army's final offensive of the war, operation Dewey Canyon II, when the Cavalry's remaining Sheridan squadrons met near disaster on the Laos border during the early months of 1971, in particular the 1/1st Cavalry. Combat field modifications A common field-modification was to mount a large steel shield, known as an "ACAV set" (Armored Cavalry Assault Vehicle), around the commander's 50-cal. (12.7 mm) gun, allowing it to be fired with some level of protection.