Although the Browning M2 has a very high strength requirement, its is the most damaging and longest ranged machine gun in-game. Only the MEC Gauss minigun exceeds it in firepower, but its ammo is likewise fairly scarce.
When loaded with rare .50 cal depleted Uranium ammo, the M2 is a particularly devastating weapon and capable of tearing apart robots, which are largely immune to other conventional firearms.
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. One mutant in the entrenchment wields the gun with ordinary rounds in it.
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. One mutant with the gun guards enter to the tunnel. Another mutant with Browning M2 stands in the center of a callow, near the final objectives.
There are several super mutants with this weapon in Kansas City. One of them snipes from the tower (opposite the West gate), other take part in South Gate assault.
Sometimes, during random encounters with super mutants, a few of them may carry this gun. Typically they have a small amount of regular .50 rounds for it.
The M2HB .50 cal HMG "Ma Deuce" has been in service with several countries for over 60 years with no plans to replace it as it is more or less perfect.
It is (in)famous for its excellent range, accuracy, and firepower. Only heavily armored vehicles (Tanks, APCs, and IFVs) can withstand it when loaded with AP ammo. For softer targets, there is a wide selection of ammunitions to meet any need.