completed when several improvements were added to produce the 90 mm M1A1, which entered production in late 1940, and was accepted as the standard on May 22, 1941. The M1A1 included an improved mount and spring-rammer on the breech, with the result that firing rates went up to 20 rounds per minute. Several thousand were available when the US entered the war, and the M1A1 was their standard anti-aircraft gun for the rest of the conflict. Production rates continued to improve, topping out in a few thousand per month. Like the German 88 and the British QF 3.7 inch AA gun, the M1A1 was used against tanks in combat but, unlike the others, it could not be depressed to fire against them. On September 11, 1942, the Army issued specifications for a new mount to allow it to be used in this role, which resulted in the 90 mm M2, introducing yet another new mount, one that could be depressed to 10 degrees below the horizontal and featured a new electrically-assisted rammer. It became the standard weapon from May 13, 1943. Anti-aircraft operation In anti-aircraft use, the guns were normally operated in groups of four, controlled by the M7 or M9 gun director or Kerrison predictors. Radar direction was common, starting with the SCR-268 in 1941, which was not accurate enough to directly lay the guns, but provided accurate ranging throughout the engagement. For night-time use, a searchlight was slaved to the radar with a beam width set so that the target would be somewhere in the beam when it was turned on, at which point the engagement continued as in the day. In 1944, the system was upgraded with the addition of the SCR-584 microwave radar, which was accurate to about 0.06 degrees (1 mil) and also provided automatic tracking. With the SCR-584, direction and range information was sent directly to the Bell Labs M3 gun data computer, and M9 director, which could direct and lay the guns automatically, all the crews had to do was load the guns. Main anti-tank developments The M3 was also adapted as the main gun for various armored vehicles, starting with the experimental T7 which was accepted as the 90 mm M3. The test firing of the M3 took place on an M10 tank destroyer in early 1943. The M3 gun was used on the M36 tank destroyer, and the T26 (later, M26)