Suppressor! "Silencer" is the term people more familiar with movies than firearms will use. There's really no such thing as a silencer and it does not work like the movies. Sure, you can achieve near silence with the right setup of suppressor and ammo, but....argh. And the inside of every brand of suppressor is different. Well, almost every. Sorry, peeve of mine.
http://www.silencershop.com/support/top-10-questions/ I wanted one for my Uzi back before I married and could spend my money the way I wanted to. The one I wanted was $1,500 which cost more than the weapon its self.
And you still have to silence the bullet. The only way I know is to slow it down below mach 1 by reducing the powder load.
When I was in school the govt was working on a super sonic gun,now they are ready with rail guns. The Russians had atomic bullets like from sci fi or so they claim http://english.pravda.ru/science/tech/19-11-2014/129079-atomic_bullets-0/ Yet we still don't have a spaceship because of the doddering fools on the hill
That's why I made the comment I did about the right combination of ammo, etc. Firing subsonic ammo through a suppressor will result with a click and that's about it. Using regular 175 grain 5.56 (for example) will just result in a nice "hiss", but still very audible (and in some cases, requiring ear protection).