A ringed barrel is easily eyeballed. That is the impact area of a second bullet striking a bullet stuck in the barrel.
Unfortunately, it is becoming more common as more people use Winchester White Box 9 mm bulk and turn to reloading due to the economy.
If you're turning up dirty patches in a barrel that seems clean it's possible someone just kept shooting a damaged barrel and it may have actually filled the ring up... but man, that's a stretch.
A ringed barrel will actually LOSE resistance where the ring is and then increase back to normal once you get past it. If you're all of a sudden hitting more resistance then it eases back up, it may be corroded or spalled in that spot and crap abraded off the bullet at that point making the barrel tighter. Bad news will be, you will never be able to keep it clean, it will always collect crap at that point.