Opened CK, selected the .esp I wanted to modify as set it as active file, looked for the gun I wanted to modify, double clicked on it and changed the base damage. After that I just saved it and that's it.
EDIT: Fixed it, it was a very silly problem. I was editing DOOM's merged weapons .esp but the gun I had i my inventory was from a different .esp before he merged his weapons in a single mod. It's hard to explain but thank you very much!!!
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