Run a search under "brass wool" on the THR search engine, that should get you some info.
I've used oil and brass wool to remove scale in the past. The upside of brass wool is that it's softer than steel, so won't scratch up the metal like steel wool will, but will still peel flakes off.
This is just totally amateur info, if you run a search on the topic, probably someone far more skilled than me will have more detailed info.
I'd just avoid using steel wool on steel gun until less harsh methods have been tried.
-MV