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Armor Piercing Rounds on Sniper Rifles only. It will shoot through one object. So if your guy ducks behind a wall- well, won't help him. Does not work with any other weapon type.
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Lag cheating, from what I've seen it also let's the people who are using it shoot you from the other side of the map through a bunch of buildings and being oddly immune to head shots.
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Armor piercing rounds, but a lot of snipers have it.