Try a workbench
If not then try using Y to repair and see if it comes up as a option to add to the weapon or something
I'm not sure because I haven't modded a weapon yet..
By the way you should of done hardcoe.. it adds so much fun!
You've been trapped in caves with deathclaws eh? Well try being trapped in a cave with deathclaws blocking all exits and you are almost dead from starvation and thirst. Now thats immersion!