Hi Nauman Marwat,
Look into progression.xml there you can find skill code and use expath or direct modification for desired effect. I am at work at the moment cant look up the code, but it may be also something about dismemberment in items.xml about each specific weapon.
I suggest using modlet for changes, easier with game updates. If you feel especially fancy you can also change description with localization.txt so it reflects your changes, but start with the code
Enjoy!