There are three ways to pick up ammo in the game
Small Ammo Packs: 15% of your reserve, not counting your current magazine.
Large Ammo Packs: 50% of reserve, not counting the current clip.
Ammo Boxes: 100% of spare ammo, not counting the current clip.
For weapons with large magazines, each time you pick up a small or large ammo pack, you lose that portion of the magazine, especially for plasma weapons.
Because plasma weapons have the same amount of magazines and ammo reserves.
In 80% of cases, clip = reserve (135/135)
So in this case:
Picking up small ammo packs = 21 rounds of ammo per gain - 21 rounds loss
Pick up large ammo pack = 68 rounds per gain - 68 rounds per loss
