I've noticed for some time (since I bought the gun a few years ago actually) that the bluing at the last inch or so of the barrels on my 16g AyA 4/53 is fading. It appears to be fairly concentric.

[Linked Image from i.ibb.co]

Yesterday, I took the gun on a flooded timber duck hunt and it got a little wet with light rain. It went into the case for the atv ride out of the woods, and I wasn't as prompt as I should have been with getting it wiped down (had a cold, wet 7 year old to take care of first-- poor boy had holes in his waders and then tripped walking out.... and still says he had a great hunt! He's the one sitting on the log.). [Linked Image from i.ibb.co]

When I got to the gun, there was a little surface rust but only on the ends of the barrels where the bluing was thin.

I polished it out with some ballistol and fine wool, and it seems fine except that I made the thinning of the bluing worse in that spot, especially on the top.
[Linked Image from i.ibb.co]

Here are my questions:
1. What causes the bluing to degrade at the muzzle? Is it just wear from casing, etc?
2. Is there something to do about this other than rebluing the entire barrel? (and being more careful about rust in the meantime)
3 When do you decide it's time to bite the bullet and do a reblue?

Jim