I wonder how a small O ring placed under the head of the screw might work? O rings come in many different sizes, diameters and material. I only suggesting this might be a possibility. I have never tried this but I would think the O ring might conform and might work. Obviously not a professional repair but might be a quick and easy temporary fix.
I also recall using a very thin cone shaped stainless shim under the heads of #8 and #10 screws where the top material under a nut plate had worn and was slightly oversized and going up another size fastener was not feasible or cost effective. Again I never tried this on a firearm but I need to see if I can contact friends still in the business and see if I can obtain a few to experiment with.Similar to what Stan mentions about making a shim.
