FMJ

Full Metal Jacket, often shortened to FMJ, is a weapon attachment that work similarly to previous Call of Duty games perk "Deep Impact". It also increases the damage by a small amount.

If a player fires a weapon with the FMJ attachment, a small rain of sparks shoots from the bullets impact point, making it easily visible for other players to notice whether or not an enemy player has the FMJ attachment. The FMJ attachment is more effective on guns with a higher rate-of-fire due to each bullet doing more damage. It is normally used with bling and another attachment so the player can both deal more damage and still have their other attachment