jtp, this is something I'd love to incorporate into a plugin I am working on, but the code here doesn't work for me (using the TP4 version of amxx 0.20)...
I added variable dump comments to every step of the changeWeapon routine and everything looks right, the ids are all correct and it gets to the point where the weapon I have _should_ be getting its clip set, but it doesnt. I even hardcoded in the number 12 for the USP I was testing with instead of using the clip lookup routine and it didnt change anything, the cs_set_weapon_ammo() just doesn't do anything for me.
What amxx platform have you tested this on? If 0.20 did you test with TP4 ? If its a matter of just installing the latest nightly build I have no problem doing that, was hoping to wait until the final release of 0.20 ...