The way I understand it, patina is oxidization. Otherwise known as rust in varying degrees. The rifle was exposed to moisture in some form, or much less likely, some chemical.
If the patina in question is not to your liking, one solution is to use a curly copper dish scouring pad and something like Hoppes #9 or a light oil such as WD-40 to remove it. I recently used this method on an especially grungy 1880 musket, and it removed the patches of rust as well as caked cosmoline. The copper will not harm the metal.
However... On a nearby thread, my gun savvy has been openly questioned, so you would do well to ask around! :wink: SW