What i'm going to explain to you is how to replace a NPC's weapon by an other one ,example: the barney's beretta by the
Jabutin's berettaFirst step:Get all the model you need, the NPC one and the new, plus a pie if you 're hungry

(if you're not send it to me).
Decompile,in separate directory, the NCP model and the p_ model of the gun ,if you don't have the p_ one decompile the w_ one, with all option check like that:
Second step:Now import the NPC's gun reference smd (ex: barney_beretta_unholstered_ref.smd) with only two option like that:

if you don't what is the smd you have to import look at the QC file with notepad and seek the gun part.
Now import the new gun's reference smd without bone or skelecton option check like that:

you should see the new weapon now
Third step:Now move the new gun model to obtain something like that:

reassign the vertex to the right bone hand (ex: Bip01_R_Hand),then delete the old weapon.
You should have this :

Now export your work as a reference smd to replace the old one, like that:
Final step:Now move the new texture in the NPC directory, compile and then

I kown i've got a very bad english so i hope you understood me