When was the 50 cal sniper rifle first used in combat?

The concept stems from anti-tank rifles which were developed mostly during the interwar period. To that end, the .55 Boys rifle could be considered the predecessor. However, these were largely unsuccessful, as armor had improved drastically during the Second World War.

There were reports of .50 M2 machine guns, in single shot mode, being used for sniping, including the (then) world record distance for a confirmed kill by Carlos Hathcock.

The first Special Application Sniper Rifle, arguably, was the Barrett M82/M107. It first saw operational use during Operation Desert Storm.