Armor-piercing round
From Halopedia, the Halo wiki
Armour Piercing Rounds or AP rounds, are the type of ammo used in most small arms weapons like Assault Rifles used by the United Nations Space Command. When all the AP rounds in a weapon are used up, some personnel swiched to Shredder Rounds, as the SPARTANs of Blue Team did on a mission in the Lambda Serpentis System.
