Q: A number of cartridges that lay forgotten all of last summer in the pocket of a damp shooting jacket have acquired some rust on the heads. Can I remove the rust, and are they safe to use?
A: That’s the problem with cartridges nowadays – the heads are not made of solid brass, but brass-plated steel. The brass plating is extremely thin, so the heads can rust quite easily, given warm, damp conditions. Rust always strikes me as being quite hard, so firing the cartridges as they are might damage the standing breech of your gun by impacting the rust into the surface.
You could remove light rust with a piece of wet-and-dry abrasive paper, then wipe the heads clean with a very lightly oiled rag.
You must never try to use WD40 to rust-proof ammunition. Experience from America, where guns for home defence are often stored loaded, suggests that WD40 can penetrate the primers and make the ammunition inert.
As for safety, either the cartridges will fire or they won’t, depending on whether or not dampness has penetrated inside. A few rust flecks won’t have significantly weakened the heads, although I wouldn’t try to fire them if they contain steel shot, just in case the shot have fused together with rust.