I haven't been able to play gmod in a long ass time but I had this problem and I solved it by removing a bunch of other weapon mods that were causing clutter, idk why it worked but you should give it a shot

having many different weapon packs tends to cause ammo type conflicts - if multiple addons define ammo type with same name, it breaks in game for all addons that use it

Imas mnogo stvari, ovaj addon mora biti sam.