OK, so it's rusted on the outside. How does it look on the inside areas that can be seen?
Does the action cycle. If so, is it easily cycled or difficult to cycle?
How does the bore and chamber look? Is there any pitting? If you can't see the rifling due to fouling, then clean it with a cleaning rod with bristle brush attachment. Also take out the magazine spring (you can find instructions online; watch a video) and see what condition the spring is in and how the magazine tube looks.
If its just a matter of external rusting then do the steel wool and oil treatment to get what you can off of it. Since it's no collector, you can attempt to re-blue it. Re-blueing products are available at your gun store. These products are far less than perfect, yet this provides some protection and makes the metal look better. Before you attempt to re-blue this puppy, you MUST get ALL of the oil off the metal or it won't take. De-oiling products are also available at your gun shop.
The rifle's lack of worth does not justify having a professional re-blueing job done on it. Too much money.
When disassembling, do NOT use a regular screwdriver with a wood-screw tip. Only use metal tip type screwdrivers, else you'll strip the slot on the head of these metal screws (wood screw threads have a much slower pitch). Instructions you can find online.
If there is significant pitting and rusting internally, take the rifle to a gun smith. It's not likely, yet there could be safety issues with this firearm, thus have it looked-over by a professional. Personally speaking, I want to keep my fingers and eyes -- they've been rather useful to me over the decades.
Let's say all goes well and you get a usable rifle out of this; from now on, when storing the rifle, make sure it's kept away from moisture. If storing it in a safe, have a heater rod inside the safe so that there will be no precipitation on metal surfaces. If you live in a particularly wet climate area, there's a lot to be said for wrapping a weapon with lightly oiled rags. A stack, or certainly a 55 gal barrel, of oily rags can self-combust. You also don't want the wooden stock to soak-up too much oil.
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