Ammunition pick-ups replenish your weapons’ ammunition regardless of their types. Grenade launchers cannot replenish their ammunition. There are three types of ammunition pick-ups.

Ammo pile: Replenishes up to one magazine of ammunition in both your primary and secondary weapon.
Ammo box: Fully replenishes your weapons’ magazines a limited number of times.
Ammo crate: Fully replenishes your weapons’ magazines any number of times.

Weapon crates containing a special weapon can occasionally be found in certain maps.
Destroy the locks to open the weapon crate.