Yep, its one of the most commmon things.
Have you tried this.
Wrap the gun in a layer of fabric cut from an old t-shirt thats been allowed to become impregnated with oil, (not soaking, dripping wet, but just "damp" evenly. Then put it in its case. That should form a moisture protectant barrier.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”