While an Absorb Shield is active, it has a set chance of adding ammunition to the user's reserves when struck with a bullet-based attack. When the shield absorbs an incoming bullet, the shield takes no damage.
As the rarity level (and therefore the material grade) of the Absorb shield increases, the Absorb probability increases.
It's advisable to equip an Absorb Shield when:
the Environment offers little/no cover from attacks;
There are various enemies that attacks using projectile weapons;
The Gunzerker uses its action skill, in order to keep shooting and being hit the least times.