slot, along with a matching modification to the missile, that solved the problem. The gun also has been criticized for having too much recoil for the vehicle weight, with the second and even third road wheels coming clear off the ground when the main gun fired. The Sheridan saw extensive action in the Vietnam War, being assigned to nearly all armored cavalry squadrons in the country. In 1969, armored cavalry units began replacing their M48 Patton tanks. The battle reports from the troops were sometimes glowing, while the reports higher up the chain of command were often negative. A 1969 evaluation of the vehicles found the M551 was employed in reconnaissance, night patrol and road clearing, accumulating 39,455 road miles and 520 combat missions, with a ready rate of 81.3 percent. Despite vulnerability to rockets and mines, it was judged worth applying modifications and equip all cavalry squadrons with the Sheridan. The Sheridan was much appreciated by the infantry who were desperate for direct-fire support, which generally served in combination with ACAVs (M113s) as armored cavalry units, consisting of both M113s and M551s as part of their TO&E. Armor units consisted solely of tanks (minus the headquarters company) and Mechanized Infantry units consisted solely of M113s. In this role the real problem with the Sheridan was its limited ammunition load of only 20 rounds and 8 missiles (though M551s in Vietnam service were not equipped with missiles or their guidance equipment, increasing the basic load of conventional rounds). 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 caliber (12.7 mm) gun, allowing it to be fired with some level of protection. The driver has an unusual rotating hatch which has vision blocks when rotated forward. Included with the set was an extra layer of steel belly armor which was bolted onto the vehicle's bottom, although only covering from the front to halfway to the end, possibly due to weight reasons. A standard modification made during the mid 1970's was the addition of the "Cereal Bowl" commander's cupola. This modification came about due to the broken rib effect that occurred when the Sheridan fired conventional rounds: the recoil would pitch the TC against the armor plating, resulting in cracked ribs on the crewmembers. Medium Tanks M48 Patton The M48s saw extensive action during the Vietnam War: over