899th Antiaircaft Artillery AW Battalion (SP) (Source: Webmaster's collection) The inside of one of the battalion's M-15A1 multiple gun motor carriages during live fire exercise at a USAREUR range in the early 1950s. The M-15A1 was based on the M-3 halftrack chassis that mounted a 37-mm M-1A2 cannon and two M2 HB caliber .50 heavy machine guns in a revolving turret (M-45). Crew of 7.