SMG

The M7/Caseless Submachine Gun, more simply known as the SMG, is a United Nations Space Command infantry and special operations weapon. There is also a version, the M7S Submachine Gun, which is equipped with an external suppressor to quiet the sound of the gunshots and reduce the flash from the muzzle. This is why the M7S is usually used for stealth operations.

Pictures
http://images2.wikia.nocookie.net/halo/images/thumb/3/3e/M7-Caseless_SMG.jpg/300px-M7-Caseless_SMG.jpg