Premium Ammo, also known as High Damage Ammo in SAS: Zombie Assault 4, adds either extra pierce or damage.
In SAS3, Premium ammo adds extra pierce and damage .
In SAS4, it gives 25% base bonus damage (rounded up), which can be increased up to 50% bonus damage with the mastery. It costs four times the amount of regular ammo. Premium ammo can also be obtained via picking up its corresponding box (it gives half a magazine for every gun you carry), or collecting any loot with the Field Supplies skill, up to a max of 100 shots per drop. It also gives the cosmetic effect of a larger muzzle flash.