I do fear that your price limitations are quite unrealistic if you dersire the best.
In my opinion, the very best 7.62X51 calibre fighting/sniping rifle is the Knight's Armament Co, SR-25. It was a favorite of the celebrated sniper Chris Kyle.
They are used extensively by U.S, Navy SEALS, and are quite popular with special operstions troops of allied armies.
Lewis Machine and tool is currently manufcturing a nearly identical rifle for British special operations forces.
IIRC, the British specification requires a rifle capable of engaging a human taget at ranges to 800 meters using standard M-80 ball ammunition.
The soldier's pack is not so heavy a burden as the prisoner's chains. Dwight Eisenhower