A few hundred M1s were 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 the low thousands per month.