+1 on the Vacuum Sealer
I have stored many firearms in vacuum Sealed bags.
I take sone good cotton rags and wash them. Then I dry them very well.
As soon as they are dry I spray them well with CorosionX or just a good gun oil.
Then I wrap the very clean and oiled gun in a oil rag and Vac seal the puppy.
Be sure that any sharp edges are covered to prevent poking a hole in the bag.
Then I toss it in a hard case to protect the bag.
Sometimes I will double bag it if it goes in a soft case.
Use a good vac sealer that pulls a real vacuum.
Your worst enemy in this situation is the bag getting a pin hole.
Last month I opened a bag that had a 1911 in it for 10 years.
This bag was in a hard case and has been in a car trunk, rented storage building, attic and even a year in a damp basement.
It looked just like it did when I bagged it.
Rifles store just as well. Just be sure and pad the firearm well so the pointy edges do not poke a hole in the bag.
I can not remember the name of the vac sealer I use. I have had it for 15 years and the bags are still for sale online and walmart.