This should be a really easy tutorial to follow.
To change a gun sound, first, open C:/Nexon/Warrock EU/sound/Weapons.
It should look something like this:
Here, you will find the sounds of all the guns in WR from A_Famas to XM8.
To edit them, simply replace the original sound file with your own desired sound.
I would highly suggest making a new folder to hold the sounds you just replaced so you do not risk losing your files.
As an example, I'll make a colt sound like the famas.
Watch the videos below for more help.
Replacing Files
Video Example
Hope I helped! Ask questions if you have any.