You get a message saying the gun was loaded but the ammo never gets into it? If that’s the case, a similar thing is happening with several types of stoves, as recorded here. I think it’s a bug with nested containers, so it might be best to go back to build 10614 or earlier, before nested containers were introduced.
Of course, that means undoing the changes you made to Generic_Guns, but an earlier version also shouldn’t produce that error. If you want to go back to 10614 or earlier, you may have to ask around on the forums for a copy or a way to get one since the experimental releases page doesn’t go that far back.