Huh, interesting.
This is a term applied to medieval armor as well.
I thought it must have been an original 14th century term, but now that I know it's applied to firearms, it makes me think it may be a general term in semi-modern use for bare iron/steel.
The armor term usually denotes a high polish though - the higher the polish, the more rust resistant the metal is, since by polishing it you're decreasing surface area.
Is that true for firearms as well?
Anyone know of any other uses?