This weapon may trace its origins back to 1938 but dont underestimate it becouse of age, The MG 131 could be a badass MG if it is triopd mounted and rebarreled to .50cal BMG.

http://img214.imageshack.us/img214/3788/mg131rq3.jpg

Manufacturer : Rheinmetall-Borsig
Calibre : 13mm
Weight : 16.6 kg
Barrel Lengh : 44â…ž in
Length : 1.17 m
Muzzle velocity : 750 m/s
Rate of fire : 900 rounds per minute
Range : 1,800 m

Quote from Wikipedia:

The MG 131 (shortened from German: Maschinengewehr 131, or "Machine rifle 131") was a 13 mm machine gun developed in 1938 by Rheinmetall-Borsig and produced from 1940 to 1945. The MG 131 was designed for use at fixed or flexible, single or twin mountings in Luftwaffe aircraft during World War II, and could be considered to be the Luftwaffe's rough equivalent of the M2 Browning machine gun.

the MG131 is far to small use for the 50bmg round. It fired a low velocity HE round. The MG151 20mm could be converted to 50bmg but it would be no easy task
Bob

More details: Wikipedia.