Although the Browning M2 has a very high strength requirement, its in-game firepower is the most devastating of any machine gun. The M2 has the longest range of any automatic weapon, but its damage and rarity of ammunition are dwarfed only by the MEC Gauss minigun.
The M2 first appears during the St. Louismission, carried by at least two guards in the super mutant Fortress in the far north of the map and two guards in the area around it. They have .50 Depleted Uranium (DU) rounds and can easily kill your entire squad.
Two mutants in a building on the right side of the Jefferson map carry this gun. Unlike the St. Louis mutants, they only have regular .50 rounds and (probably) won't tear your squad apart with a single burst.