Use a Zip program
zpak000_assets>models>weapons2>*weapon you want to change*>*weapon you want to change*_hold.md3
Then from there start editing the animation and weapon cycles
look at the animation.shader for the animation frames.
Then export it to .md3 and overwrite the other one
It will change the weapon from your view point(when you hold it)
P.S. you prob wont be able to get on other servers with this
Other P.S. Textures for the weapons are in the same folder
.