Dear JWR:
The use of military surplus ammo boxes as Faraday shields was recently mentioned again in SurvivalBlog. But readers should be reminded that these cans will not work in the configuration where they are normally purchased. This is because the boxes have a rubber gasket to seal the lid from water and that makes the lid not in [electrical] contact with the body of the can, thereby losing the [EMP] shield effect. Regards, – Dave X.
JWR Replies: You are correct. As mentioned previously in SurvivalBlog, the best approach is to remove the rubber gasket, rough up the metal on both the top edge of the can lip and in the gasket groove (with sandpaper, a wire brush wheel, or a Dremel tool rotary stone) and replace the thickness of the gasket with stainless steel wool which is tacked in place with small globs of epoxy at two inch intervals. (NOT a continuous seam of epoxy!)