Normal M4A1, only without a built-in sight.
Something like M4A4 from CS:GO. Very comfortable, the gun will loom before my eyes, and close inspection of the inspection better. But to control the return was difficult (personally, I found it difficult to control attacco without sight).
There is one drawback. Because Valve hacks, they did the M4A1 is detailed on all sides, but only the one the player looks. Due to the fact that in CS:S there is no animation of some weapons like in CS:GO, and CS:GO was originally the weapon was fully done. Now, therefore, the M4A1 is a stub. And it shows in the screenshot. But fear not, it is not so evident!