If you're talking about Korea and American issued firearms, then it's not going to be the American M-1C or M-1D Garand. The M-1C had anything from a Lyman Alaskan (adopted as the M81 or M82, depending on its reticle) or if the M-1D with the somewhat better M84 scope (more rugged and a little better sealed against the elements). Those scopes had low magnification which limited their usefulness. The USMC rifle, a M1903 Springfield with a 8x Unertl had superior optics and was a better gun for longer reach. I'd stay away from the Army's Springfield (M1903A4) with its miserable Weaver scope (adapted as the M79).

For overall ruggedness, reliability and strength of optics, I'd go British with the No. 4(T) in .303 Enfield. It was basically a selected SMLE fitted with that heavy telescope. They were still around during Korea and were replaced by a similar gun in 7.62 mm Nato (with shortened forearm and handguard).

Col. Brophy did experiment with a .50 cal barrel adapted to the Russian anti-tank rifle. He was able to score hits at 2k yards with it. I don't know if he ever tested that gun in combat though.

BTW, you should check into Peter Senich's series of books on sniping. Ian Skellerton has one on the British rifle.