Thanks for the info/reply and now some followup...
Talked to my friend and seeing he already had some Bar Keepers Friend in the kitchen sink cupboard he gave it a try first.
Said it worked great, gun cleaned up really nice with it. He's planning on selling it (he thinks he already has a buyer) so we won't know about any long term problems it may have caused...
Many thanks to everyone who commented/helped with this :)