That's an M4 with an aimpoint, not an M16. You are probably looking for it when playing as a rifleman. It's a weapon for USMC sergeants, so choose the sergeant position to get it. You can also choose an M4 with ironsights and a vertical grip as a sergeant. Some classes, like sergeant, have multiple weapons. To choose between them, use the weapon selection menu right above your character's picture on the class selection screen.
I snipe better with a shotgun than a scoped rifle. =/
