Topic on Talk:Shadow Man
The 2015 Texture Pack Update includes the new Shadow Gun sound effect.
It's stored as: "\data\audio\sfx\weapons\shadgun\sgfire1 (Tims special).wav"
There are no instructions, but I assume that you have to rename it to "sgfire.wav" and replace the original file.
I don't know what the "stability patch" was - I can't find any information on it.
However it seems that the Steam version (and GOG?) was patched, so perhaps those fixes are now included.