Don't rule out Marine grease.
I use the Super Tech (Walmart) brand on my U.S. RIFLE CAL. .30 M1's.
I also run a jagged patch of grease down all my bores and wipe down the metal when I put my long guns up for the Season.
Sometimes they end up sitting for years.
It's only a few dollars ($5.00, I think) for three little tubes or one big one, and because it is Marine grease, it is very sticky so it films really well and is also loaded w/ industrial rust and corrosion inhibitors.
And because it's grease, it doesn't evaporate away.
Cleans off easily w/ CLP or Hoppe's and a rag or patch, although a little residue is OK too as it provides added protection from the elements in the field.
Highly recommend it.
“...Cogito, ergo armatum sum..."
(I think, therefore I am armed.)
-- Lt. Col. Dave Grossman --