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Maybe you need a non-leather holster that covers that area. Leather retains moisture where the plastic types allow it to dry. Any time I am in a humid area I use a plastic holster. It keeps the gun dry even though it is not as comfortable and doesn't move with me as well as leather. Another thing I do is carry a towel and silicone cloth to wipe the gun down and then protect it with the silicone cloth. Maybe, if you don't already, you can use a silicone cloth on the mag release and see if it helps. I don't know about your gun but it sounds like a moisture retention problem that lasts for long enough to cause rust where none should be. Also, you should store your gun outside the holster if you don’t. Those plastic ones are good for that too.