Pretty much the same process as the tutorial Tomsen1410 posted, just open up a GDT and edit the multiplayer version of the weapon.
1) Open Asset Manager
2) File > Open and navigate to <root>\source_data\
3) Open up the GDT file for the weapon you wish to edit
4) Select the weapon variant from the list on the lower left of Asset Manager
5) Change values, then save file (make sure the GDT isn't Read-Only, shouldn't be)
6) Navigate to PC Convert > Single (with the variant still highlighted)
7) Navigate to raw/weapons/mp and look for the weapon you changed (Fast way is to sort by Modified date)
8) Copy to your mods weapons/mp folder
9) Add to IWD (check it in the IWD list) and FastFile (zone source add weapon,mp/<name>)