The simplest form of protection is a thin layer of grease, be it axial grease or gun grease. Lightly coat the metal parts with with the grease. Run a lightly coated patch through the barrel.
Do not plug the barrel as you may be sealing moisture into it.
REMEMBER you will need to clean the barrel before use.

If you are using a steel safe you can fit a heat source to keep the air dry and help prevent condensation.
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But honestly a good, clean and oil before storage should see you through months of storage. I'd have my O/U in storage 6-7 months of the year before pulling it out for a bang. After August early September my Semi only sees a few harsh days out before being beasted over the spring and summer months The same goes for one of my rifles . Out for September to February and then back into storage for the next 6 months.
Taking it out of storage every now and then to give it a wipe will give you something to do.

Go to any camera,computer,HiFi TV shop,when they re open and ask them for any of the little silica dessicant bags out of the waste packaging.Easily spotted by the DO NOT EAT labels on them. Just take a few of them and drop them in your gunsafe as is.Or if you have some sort of little cloth bag ,empty them into the cloth bag ,until full and hang that in your safe.Its just as good as those dehumidifiers.

For grease,if you can get it,the original gun Cosmoline is still the best around.
Next,is simply vaseline,which can be melted in a pot of boiling water and applied liberally to your metal parts.
I'd be careful of using any of the mentioned bearing or axle grease or even vaseline on wood or some plastic parts tho. It can discolour the wood and be absorbed into the wood itself.Truning it black in some cases.Reason many old military rifles are so dark in colour,ages of being in grease wrappers leached into the wood and reacted. They can also damage some plastics as well if you have a stock made thereof too.

Bit of a different one here, my .22 is beat to hell and back was like that when I bought it, it's blueing is faded out across the barrel and the only thing that has kept the rust off the barrel when it's stored for extended periods is ... sunflower oil and sometimes linseed oil , I apply it with a rough kitchen paper towel , great way of cleaning rust spots off a gun too it immediately seals the pores if the metal after you wipe it down

cookimonster wrote: »

Be carefull of Linseed Oil, its not recommended for oiling guns. I had to look up the technical qoute for it:

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While vegtable oils may serve a purpose for external oiling for storage, many of them are not suitable as lubricants as they break down too easy. Tallow was a favourite back in the day, but again I'm not sure as to its suitability as a lubricant on modern firearms.

3 in 1 oil is what my guns get for the inner workings , never had any issues with it i run my guns as dry as possible only oiling small areas that need it
And as for the linseed oil creating a coating thats on purpose for me put it on when the barrel is toasty and it will blacken it which helps hide scuff marks

tudderone wrote: »

3 in 1 oil isn't what it was, it dries out and leaves a gummy residue that gums up whatever you have used it on. I was given a set of micrometers, that were good quality but dirty, so i dismantled them and cleaned them in acetone. When reassembling them i used 3 in 1. Used them a few months later and they were all gummy and stiff. I had to redo the lot and used watch oil.

Never had that issue, though i clean it every 500-750 rounds which includes total wipe down and then reapplication of oil
Shotgun also gets 3 in 1 oil
For my centrefire guns though i use Hoppes lubricating oil the little orange bottle