The 12 gauge bore in question is from a single-shot gun that has a strange spot on it that almost looks like it was etched out, although it might just need to be cleaned. It almost looks like it has a little "island" in the middle, and I don't know if pitting would do that.
I used a bore brush (all I had was a 20 ga brush) on this gun, but the spot would not disappear, so I assumed it was some kind of pitting. The gun was given to me, and it only has a light amount of rust in a very small spot or two, but otherwise the barrel looks good. I noticed the spot because I had bought some Estate brand shells, and had read that they shoot dirty.
Thanks for any help!