In response to your statement that "It looks like chatter from a reamer"... I will dis-agree with you. The marks are radial in nature. This is an indication of a single point tool. (Boring bar) Or a chipped reamer. If it where chatter, the marks would be axial in nature and look like a washboard going around your forcing cone, not a ring. If you have a lathe or a friend with a lathe... Turn up a brass cone able to be driven by a bushed cleaning rod from the muzzle, and lap away. I say lap, because it is slow and needs to be checked often. Reamer will be quicker but more dangerous.

Being a machinist, I am too stupid to be afraid of ruining my guns. And too proud to take them to a smith. If you are not one of the above... Don't be like me.