Hey, so these sounds are directly ported from Red Orchestra 2 and replace
the firing sound for these (most of the Russian/German) weapons in
Siminov's "All WWII DOD Sweps V3":

- PPSH-41
- C96
- DP-28
- SVT-40
- K98
- MG-42
- MP-40
- Mosin-Nagant
- P38
- TT-33
- G43

Thanks to Siminov for making the original weapons pack in the first
place ( http://www.facepunch.com/threads/1134695 ). Also, thanks to
Tripwire Interactive's development team for the sounds
( http://tripwireinteractive.com/ ).

To install, just take the "All WWII DOD SWEPS V3" folder, and move it
to "steam/steamapps/username/garrysmod/garrysmod/addons", and if it asks
you to merge some folders and replace some files, click Yes. And don't
worry, I included a backup folder with the old sounds under
"sound/weapons/backupfire", so if you want to rollback, just copy from
there and paste over to "sound/weapons".

- Highdef

Also, here's the pack:
http://www.garrysmod.org/downloads/?a=view&id=126013
http://www.garrysmod.org/downloads/?a=view&id=126014