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.
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!