The M15A1 was based on the M3 halftrack chassis and mounted a 37mm M1A2 cannon (200 rounds and two 12.7mm M2HB heavy machine guns with 1200 rounds.
These vehicles were assigned to AAA AW Battalions (SP) at a rate of 32 plus 32 M16 halftracks per Battalion.
The Cherbourg AA defense scheme called for 3 Battalions of heavy guns, 2 SP battalions with their M15 and M16 halftracks mounting automatic weapons and a battalion of searchlights.
http://www.antiaircraft.org/M15.htm
http://www.antiaircraft.org/M16.htm
The Luftwaffe was relatively active in the immediate post D-Day period, although any attacking aircraft had to run the gauntlet of a comprehensive fighter screen, not to mention frequent attacks on their own airfields. Even in the night time hours, the beachhead was well served by RAF Mosquito nightfighters that took a heavy toll on Luftwaffe intruders.