The Browning M2 is an air-cooled, belt-fed machine gun, which fires from a closed bolt, operated on the short recoil principle. It is perhaps the finest heavy weapon in the game, offering massive firepower with each burst that's only supercharged with the .50 depleted uranium rounds, and a range that's just five units shy of the sniper rifle. The only problem is the high strength requirement, which can be mitigated with power armor and/or Buffouts. The only weapon more damaging than it is the MEC Gauss minigun, which is mostly a curiosity too expensive to actually fire.
St. Louismission: Carried by the perimeter guards in the bunkers in the mutant fortress to the northwest. They carry depleted uranium rounds, which are likely too much for the player's squad to survive, so they will usually have to be plinked to death from beyond the M2's range.
Jefferson: Two mutants on the east side of the town carry the Ma Deuce, loaded with standard rounds. Easier to get by a long shot.