Best way to get out Stains on Guns?

[marklbucla]

What's the best way to get out the stains that appear on the older guns? I tried Hoppe's, CLP, MPro, etc. and none of it worked.

Here's a pic of an example. Check out the underside of the carrier.

 

[Brian Williams]

Not really stain, but a patina that the steel has developed. Basically it is a rust that is not really involved, best way to remove it is to run it thru a buffer and polish it off.