Carbon build up??

[Lee Enfield]

your pictures look to me like corrosion and pitting - a so called "dark" bore.

If you want to try to remove carbon, go to your local hardware store and buy CLR -

wet patch with CLR, but do not leave it in for an extended period.

Information

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[browningautorifle]

corrosion and pitting

Kind of what I was thinking. With all you've done, it's all that can remain. Yes, I had one exactly like that. Can you get your hands on a borescope for a real look inside?

[ssgross]

wet patch with CLR, but do not leave it in for an extended period.

do be careful, CLR works great but will remove bluing and some other finishes (don't ask how in know). When I have a rifle needing CLR, I tilt the bore slightly down, and work from the muzzle end. Keep towel handy and continually dry the muzzle as you patch. It only takes a few minutes for bluing to wear off the crown if you aren't careful.