Eric, the area of the barrel with finish loss in the photo you posted isn't what I'd refer to as a "halo".
The FAQ Document has a good example of what a "halo" looks like on page 58.
A number of things can reduce the blued finish (which is a magnetite state of iron oxide) back to steel, or physically remove the very thin layer of bluing. It can be done chemically or through abrasion or galling. There are common household chemicals that will reduce blued steel back to bare steel.
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 Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.
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