this is the most vague part in the game Look in your weapons tab in the pipboy, select the weapon and look at the upper right hand corner of the pipboy screen, showing X(defaultkey) and next to that "mod", Hit the key on your keyboard and a mod menu appears to confirm the weapon mod. I could not find out how to do this when i wanted to try it out, i googled all over for the steps in doing this,nothing... and plus the only Derp(Chet or whatever) in the game who has any information on weapon mods, is totally worthless, "what are weapon mods and special ammo?" "weapons mods are thing like blah blah blah and i cannot reveal how this is done but lets move very elaborate like about how to craft ammo!" F**k that guy, matter-fact after this im going to play one last time tonight and just going to harm him and hit f9 and repeat! goto pipboy, click on gun, Hit X and there ya go, good huntin!