M7S SMG
From Halopedia, the Halo wiki
The Silenced SMG is a standard M7 Caseless Sub Machine Gun (SMG) with a silencer attached to the barrel, making it less noisy and more accurate. It also has a laser sight.
This weapon is only ever 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!".