An American/Belgium assault rifle. It is currently used by the USMC and the US Army in Afghanistan. It has also been featured in Modern Warfare 2 as one of the American baseline weapons.
A perk in the Call of Duty series. This perk has only been used in Call of Duty: World at War and Call of Duty: Black Ops. It reduces explosive damage.
Damn, my grenade didn't slaughter that guy because he had Flak Jacket as one of his perks.