Hi Kevin,
Residue from cap smoke and fumes will cause pitting in steel very quickly if not completely cleaned away after firing. Pitting will lead to rust soon after.
Zinc Alloy also seems to suffer from over exposure to the same residue so the most important task is to thoroughly strip and clean your modelguns. Once that's done, ensure they're dry and give them a good coating of either Silicone or Gun Oil.
My own personal preference is to use Silicone on the abs and HW models, and Gun Oil on the metal models.
For long term storage I tend to oil the metal models liberally wiping them clean again prior to use.
Depending on the gun, it may be worth considering taking the recoil spring out so it isn't stored too long under tension.
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Cerwyn
Hobby collector of Replica model guns and Militaria.
also member of Living History Reenactment Groups.