1. Go to res_mods/0.x.x/scripts/item_defs/vehicles/[nation of the gun that got the desired sound]/components/guns.xml
2. Search for the gun that got the sound that you want. If you can't find it, go to res/scripts/item_defs/vehicles/[nation of the gun that got the desired sound]/ and open the [tank of the gun].xml with WoT-tools, and search for the line where it specifies the guns and see what it's called, so you can find its string name in the guns.xml. You can get WoT-tools from here: http://www.mediafire.com/?a0s5np8s5yfalk5
3. Find shot_*x* and copy shot_*x*
4. Do exactly as step 1. and 2., except that this time you may want to find the gun that you want to change.
5. Find shot_*y* and replace shot_*y* with shot_*x*
6. Then you are done!