I've been thinking about it and it comes from the inside. It looks like it was the barrel maker who made that ring. I suspect that the barrel was rifled by a button instead of a single tooth cutter. The button is a cutting die that is pulled through the bore.
Hold the bore up to the light and see if the bore is clear. Look for lumps.
Bore looks great
Doesn't a button displace the metel and not cut it ?
Today is my Best Day.