There are 3 types of .30cal: a single magazine, two magazines and an ammo box. The one magazine gives you 2 rounds, the double gives you 5 rounds, and the ammo box gives you 10 rounds.
The magazines are similar to the WW2 surplus ammo belt for M1 Garand, the ammo box is white and unique to this game.
This is the rarest type of ammo in the game. So use it wisely.