As a military abriviation, this is short for Armour Piercing. As in an Armour Piercing round (bullet) or rocket.
An AP round is designed to penetrate through protctive armour, most AP rounds can even penetrate level 3A protective armour (but may have trouble with gunshield
s) AP rounds are nicknamed "greenies" by the guys in my PLT
because of the dab of green paint on the tip to indicate it's an AP round.
An AP rocket (for example, an AP FFAR
) is designed to pentrate the armour of heavy armoured vehicles (tanks) as a sole purpose.
"We've got multiple suspects in paramilitary gear, full body armour, so everyone load up with AP rounds"