Short story: I created (remade) these sounds a few years ago. Because the original sounds of weapons from VALVE I did not like - hissing and indistinct purring. Creation, remade, testing took that half a year. File after file.
Now the game can sound sharp, distinctly, realistic! Of course, if you download this mod...
Links:
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For nonsteam players [You must be registered and logged in to see this link.]
How to install: Download "l4d2_realistic_weapon_sounds.vpk" and copy to (path to steam folder)\steam\steamapps\common\Left 4 Dead 2\left4dead2\addons\
Warning: If you use mod (skin) weapons with a built-in sound, then the built-in sound will be played!
p.s. special thanks to ⓜⓨⓛⓙⓐⓒⓗ for helping with "Authoring Tools" and other thing!