People call this gun M4A1 Carbine with Silencer in full. Usually favoured by Counter-Terrorists, the police, and the SAS. Here's some details I remember last time I used this nice trendy gun.
Ammo Per Clip/Ammo Held in Weapon: 30 rounds
Max Reserved Ammo: 90 Rounds
Special Weapon Function: Attach/Detach Silencer
Ammo: 5.56mm slugs (30-round mags)
Reload speed: Medium
Cyclic rate: High
Overview: One of the most kick-ass guns in CS, you'll find yourself using this a lot. It's stealthy, semi-accurate, and packs quite a punch. The silencer is optional, but does decrease damage it deals with the prize of making yourself quieter, obviously.
Personally I think this gun is very cool. One of the most commonly used in Counter-Strike. When fired, it rattles like hell. But put on the silencer then you've got cool pu pu pu pu sounds.
This gun was used by Sarah Connor in Terminator 2 when she was assassinating Dyson. Also, Arnold Swartsinigger used it on the T-1000, pummelling him with M4A1 bullets in the liquid nitrogen truck.