If you give the gun to your own character do they equip it? I've just successfully made a new gun and originally found that while the correct gun was being equipped, it wasn't loading the model. What fixed the problem was to create and set a "1stPersonModel". In the stat sheet for the weapon there is a drop down box where you set "first person model". The first person model is a simple static (filter for "1stPerson" in the statics list is probably the easiest way to find the existing ones) which should be pointed at your new gun model, then you can pick it in the drop down list on the weapon screen. Apologies if the above is a bit vague, I'm not at my own PC so can't check the exact details in the GECK. Also, NPC will use the best gun in their inventory, so it may be worth setting the damage to an obscenely high value just for testing.