qnli question: How many of the M2HB guns were mounted together on the M45 Quadmount weapons system? sentence: Their needs could cogently be met with smaller-calibre ordnance beyond using the usual singly-mounted M2 .50 caliber machine gun atop a tank's turret, as four of the ground-used "heavy barrel" (M2HB) guns were mounted together on the American Maxson firm's M45 Quadmount weapons system (as a direct answer to the Flakvierling),which were often mounted on the back of a half-track to form the Half Track, M16 GMC, Anti-Aircraft.