Why would the U.S. go for a 300 magnum round instead of recalibrating the 7.62 adding more grain?

U see the army and Marine Corpse chose the 7.62 bec it was easy to carry and deadly effective (not accurate, bec. that is due to the rifle) out to 800-1200 meters. Most snipers use either 7.62 or .50 either m24, m21, Remington 700, m82 (new edition) and m107. IF the logic is to sue a round with more velocity and less speed lost in midair, why not increase the grain # Therefore it would not be as heavy as a standard supersonic, FMJ (.50) round. Yet it would be as effective as 0.50 and could travel greater distances and deliver enough kinetic energy to literally make a melon sized hole in the guy.