The Silenced SMG is a standard M7 Caseless Sub Machine Gun (SMG) with a silencer attached to the barrel. All silenced firearms sacrifice accuracy, power and range in return for greater stealth. It also has a laser sight.
This weapon can be seen on an example map included in the files of Halo 2 Vista's built-in map editor. The map is simply named "Example" and the map's description is "This is just a small example of what you can do as a Halo 2 cartographer!". The silenced M7 can also be found in the Halo Graphic Novel and in Halo: First Strike where SPARTAN-117 used two Silenced SMG but only used his Battle Rifle during Operation: FIRST STRIKE.