The official M4A1-S Description states:
With a smaller magazine than its unmuffled counterpart, the silenced M4A1 provides quieter shots with less recoil and better accuracy.

The M4A1-S is a Counter-Terrorist exclusive rifle in Counter-Strike: Global Offensive and was added in the game on the August 14, 2013 Arms Deal update. It is a variant of the M4A4 assault rifle, replacing it if equipped in the loadout. It is a successor of the M4A1 from previous Counter-Strike games.

The M4A1-S in game is based off a Colt Model 723 “M16A2 Carbine” with an “M4” profile barrel.

In game, the rifle has 20 rounds per magazine and 40 rounds in its reserve. A new feature implemented is that the silencer now takes more time to attach and detach and it is already attached once the player purchases it. It’s rate of fire is slightly lower at 600 RPM. Otherwise, it works identically to the M4A1 found in previous games.

Popular CS GO Skins for the M4A1-S:

M4A1-S | Hyper Beast

M4A1-S | Hyper BeastM4A1-S | Cyrex

M4A1-S | CyrexM4A1-S | Chantico's Fire

M4A1-S | Chantico’s FireM4A1-S | Mecha Industries

M4A1-S | Mecha IndustriesM4A1-S | Golden Coil

M4A1-S | Golden Coil

The magazine model of the M4A1-S depicts a curved 30-round 5.56×45 STANAG magazine (also used on M4A4), despite still having a 20-round capacity.

Statistically, the CS GO rifle is more popular than the M4A4 despite the magazine reduction, arguably due to its silencer and more controllable recoil.

The noise-reducing effect of the silencer is somewhat exaggerated. Though in-game shots are depicted as audible but quiet “pew-pew” noises, in reality a silenced gun would still produce 130-145 decibels of noise, which is still as loud as a jackhammer.

The weapon depicted is actually not a M4A1, but a Colt Model 723, a early predicessor to the M4 Carbine. This is identifyable due to the non-removable carry handle, while the M4 has a removable carry handle.

Hitbox Primary Attack
Unarmored Armored
Head 132 92
Chest & Arm 32 23
Abdomen & Pelvis 41 28
Leg 24 24
Over 100 signifies a guaranteed kill.
Weapon Finish Rarity Minimum Wear Maximum Wear
M4A1-S Atomic Alloy Classified 0 0.9
Basilisk Restricted 0 0.68
Blood Tiger Mil-Spec 0 0.3
Boreal Forest Industrial Grade 0.06 0.8
Briefing Mil-Spec 0 0.8
Bright Water Restricted 0.1 0.22
Chantico’s Fire Covert 0 0.99
Cyrex Covert 0 0.5
Dark Water Restricted 0.1 0.26
Decimator Classified 0 0.85
Flashback Restricted 0 1
Golden Coil Covert 0 1
Guardian Classified 0 0.5
Hot Rod Classified 0 0.08
Hyper Beast Covert 0 1
Icarus Fell Restricted 0 0.1
Knight Classified 0 0.1
Master Piece Classified 0 1
Mecha Industries Covert 0 0.8
Nitro Restricted 0.06 0.8
VariCamo Mil-Spec 0 0.6