The firing sound is derived from S.T.A.L.K.E.R. Shadow Of Chernobyl, and the reload sound is from a combination of both Stalker and Fallout.
Installs like any other mod.
The new sounds can be heard here:
https://www.youtube.com/watch?v=zVS-gspoufo&t=5m38s
https://www.youtube.com/watch?v=vNK4rF7CLFM
https://www.youtube.com/watch?v=6bdSKpZ1GRY
Special thanks to Gopher, tehgrandpa and pixelgamerch for featuring my mod in their videos.