I think you may be a little confused. Rifles don't have ballistic coefficients, bullets do. A ballistic coefficient is determined by the weight, diameter and length of the bullet. It determines the aerodynamics of bullets and allows us a means of calculating trajectory based on the velocity of the bullet.
The bullets used in military weapons have ammo that is good for its intended use. Most troops are doing battle at 300 yards or less, they don't need long range ballistics. Therefore 5.56 is sufficient even with low b.c. bullets. On the other hand snipers that are engaging targets at 1500 yards need the high b.c. of the 338 bullets in the 338 Lapua.
Hope this clears things up a bit