Hi,
This is a hungarian sniper rifle from the Gepard anti-material rifle family. This verison is the Gepard M4.
For more information:en.wikipedia.org/wiki/Gepard_anti-materiel_rifle (english)
hu.wikipedia.org/wiki/Gep%C3%A1rd_M1 (hungarian + pics)
(I am not allowed to send url's)
I think some of them was transferred to US in the 90's.
It's not .50 caliber, but 12.7 mm (russian).
Bye,
GZsolt