Once "scale" like this forms on a metallic surface, there really is not much of anything you can do, except to maintain the present condition and guard against further deterioration. Although there are chemical strippers that will remove the surface rust, they cannot restore or replace any metal that has already been lost, and if applied, they will not only remove the rust, they will remove any bluing (which is also a form of rust) and may even etch the surface. In fact, the finish on your rifle's receiver looks like it may have been tin- (if not nickel-) plated, so those areas where the plating remains will continue to be protected but not without some personal assistance (i.e. regular maintenance). Removing the rust would be akin to replacing all the wood with new wood, or replacing all the screws, etc, with new ones, such that in time, your rifle would cease to be valued as a rifle in its original condition but a rebuild. Like Jason's ship (from the Argonaut's of Greek myth) was preserved for some 300 years. Every time a timber was found to have rotted it was replaced, such that in time (after 200 years or so), all the timbers had been replaced, and while the ship looked like the ship the Argonauts had travelled aboard, it was not the same ship because all of its original parts had been replaced with new-made parts. There was nothing left of the original ship except for the legend.