If you have the large EMP grenade (not the one that can go on SCARS or FY71s), click and hold it between three to five seconds when there is an enemy near. You will be told where to put the pointer to get a direct hit. This is not a special attachment or special EMP Grenade Launcher. You can only get it at one point in the game. It can be an EMP Grenade Launcher that falls out of the sky, as long as it is normal and not broken.