You can attach a sound suppressor (silencer) to Assault Rifles, Pistols, Submachine Guns and some Snipers. You can try just working on this anywhere you want, but in the end Presidio comes out on top as the best spot to nab the achievement. Set up a terrorist hunt on any difficulty on Presidio. Attach your silencer by holding X and pressing LEFT on the D-PAD. From the start, go straight ahead, open the door on the left then turn around and quickly duck behind the pile of boxes that juts out from the wall. Hold UP on the LEFT THUMBSTICK so you're up above cover and wait for the enemies to start pouring out of the door you just opened. Aim your crosshair at head level and fire in bursts... you should get this on your first try.