How to store a firearm for a short to medium amount of time and protect it from rust?

Your going to get a 100 responses. IMHO clean the bore with some solvent and then oil the bore. Wipe down the outside to get any sweat off with a good product like G96 Complete Gun Care spray. Store it some where reasonably dry. If your shooting a milsurp with corrosive ammo run some patches down the barrel with a "windex" type product. It's mainly water with a little soap in it. That will dissolve the salts from corrosive primers. Then clean as above. not rocket science and doesn't take all the magic concoctions available today. One of the top ones is basically coconut oil thats available in your local store cheap. But not in the cool tactical bottle. Pretty simple really. Good luck.

Oil can damage wood grips and stocks over a period of time, otherwise I would just spray the gun with a good CLP and be done with it.

Coating the exposed metal with a thin film of grease may not be elegant but it is effective and wipes off easily.

I'd give my cosmo to Rachel any day.

I wipe my gun to store in the safe with Barricade---I check them every 6 months & wipe again. I have used this product for many years before they changed the name & have never had any rust on my firearms. Great product.

I use whatever motor oil is on sale at auto zone, applied to metal surfaces with a small paint brush. It works here in Fl, which is not known for low humidity. And everywhere else I have done it. And it's cheap.

A good coat of whatever oil you have that doesn't run off, or that leaves a coating. I was partial to the way oil I used for my lathe, when I had it. It didn't have detergents or anything to make it spread. Helped that I couldn't find it in anything smaller than a gallon, and two machines run in the evening by one person will take a long time to use a gallon.
Heck, I bought a gallon can of WD40 to clean grease and cosmoline off tools, and that works fine for anything short of long-term.
Otherwise...

65ml for $15, I put a double- and triple-thick coat on every gun and knife I own, all of my dad's 32" double-barrel, a handful of collectables and heirlooms, a bunch of tools, and anything else metal I don't want corroding. So far it's worked on wood and leather, too, and I have yet to find a plastic it reacts with. More careful with that, though.
No rust yet and I still have half a can.

A brand of The Safariland Group, MIL-SPEC Break Free CLP Liquid https://www.safariland.com/news-posts/mil-spec-break-free-clp-liquid-now-available.html

Exceeds the MIL-PRF-63460E(4) military test standards

Wax for storage. Oils are fine but needs to be reapplied every so often. Here is the reason. Oil will rub off much easier then wax when handling your guns. Dust will stick to oil. Once enough dust is collected, moisture will collect on the dust. Dust does not stick to wax.
Renaissance Wax is great stuff, but you can get by with standard paste wax like Minwax.

Any oil will do. Eezox in a climate controlled area works for me. I have many guns that go 3-8 months or longer without being touched. Never had a speck of rust.

http://www.nramuseum.org/gun-info-research.aspx NRA Museum uses wax.

Once it is cleaned & oiled/wiped-down, I would not worry about any of my firearms "living" outside of my gunsafes in my house for months or years ... and several of them have ... and none showed any rust issues, only dust issues.

Years ago I took steps to assure that the RH in this house never passes 60% and that there is constant air movement.

The basement experiences the highest RH and I have had several rifles on open racks down there for years between cleanings/wipedowns and none of them show any rust formation.

I have been using Eezox for 10-12 years, but prior to that just whatever gun oil was handy at the time.

FWIW ...

======
Just checked ... the M1917 Eddystone that I bought in Aug'10 has been sitting in the external basement rack since I received it. All I did was clean & very-lightly-Eezox the thing and place it in the rack until I had time for a detail disassembly/inspection. (Yeah, I'm meaning to get around to that when I get a moment. ) Still no corrosion showing under the dust.

Long experience has proven to me that wax is better than oil for preventing rusty firearms. Johnson's paste wax in the yellow can is perfectly adequate and a can will last you almost forever unless you have a tremondous amount of guns or go really crazy about waxing them. You can even clean a bore and pull a waxed patch through it a few times if it's going to set for some time.

VCI Bags.... very,very effective and super easy.

http://www.polygunbag.com/gunbags.html