I see a lot of people using it; and on guns its the normally the last attatchment to be unlocked. Please tell me what it does and why its so awesome?

It does no extra damage when firing at a target out in the open, but rather is intended to help kill players behind cover.

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. It also changes the sound of the weapon, primarily the sound of impact. The easier way to acknowledge a FMJ user is the impact effect when the bullet hits something, it glows a bright yellow spark instead of discharging normal dust. It is normally used with bling and another attachment so the player can penetrate cover more easily and still have another attachment.

There is no added damage. It is exactly like deep impact from previous titles. Your shot looses less hit points while passing through objects. (note that rapid fire attachment is the same as double tap)

What's so awesome about it? Nothing really. Unless you liked deep impact. I, however, did not. But now rather than having a wasted perk that is never used it is merely an attachment.

are u serious...even when u put FMJ on it shows the damage increase...so its pretty obvious that it adds damage

You guys are forgetting one of the best reasons for using it. It allows you to shoot through a riot shield.........