The potassium chlorate in corrosive primers is the culprit, not the propellant. Upon firing, it is chemically transformed into a form of potassium chloride, a close cousin of common table salt.
Humidity then causes the bore to rust.
The tar-like coating around the case mouth on some military ammo is a sealant against the intrusion of moisture. There are commercial preparations and home brew recipes to make such a sealant that can be applied to the juncture of the case mouth and bullet. They can be applied with a Q-Tip cotton swab for use in wet and rainy hunting environments.