extended mags are and attachment for your gun when you get 40 bullit penetration kills with fmj
get 30 FMJ Kills
You have to get a certain amount of kills while using FMJ (Full Metal Jacket)
it gives you more bullets
Play a private match with 1 or more players, go to the opposite sides of a wall with FMJ on any weapon, and shoot each other 40 times. You MUST be in a PUBLIC match to get any attachment