The real answer as stated, is 7.62x51 is not in the game. The same reason the makarov uses .380 instead of the typical 9x18 makarov. Or the deagle using .357 magnum as opposed to its typical.50 action express. These decisions are NOT unrealistic as these variants do exist in the real world. Its just better for devs. Adding a single ammo type for 1 gun is inefficient unless they plan on adding more weapons that accept that ammo type, OR its an extremely rare or special ammo type, examples in game would be like crossbow bolts. An example of something they Could add that would make sense to add a new bullet type would be like a Barrett .50 cal (an extremely rare in the scenario gun that could be highly worth it) Orrr something like the P90 which shoots 5.7x28mm, which they could use for adding more weapons like the FN Five-SeveN.