Evapo-Rust
http://www.evaporust.com/
I'd call them and check before putting a blued gun in it though, just to be safe. There is a story online of a guy who documented the abuse of his Glock then buried it in the garden for a few years. He has pictures of it coming out of the ground and then after spending the night in Evapo-Rust. It was amazingly effective.
Keep steel wool away from your guns. Small pieces of it break off and embed themselves and then rust quickly. Use bronze wool instead if needed.