Rings around brass...

[Webster]

If it was the die it would be more like a single smear, not rings. The die is burnishing the tops of rings on the brass formed in the chamber.
3 rings, equal spacing leads me to believe 3 passes with the reamer. Probably chip weld on the reamer.

I don't think it was made clear if the rings appeared after sizing or after firing? I don't know anything about machining. If a chip was caught between the reamer and chamber wall, would it make a spiral ring as the reamer is moved. What's the feed rate of the reamer? Do some gunsmiths run the reamer in the bore hole or drill the chamber area bigger so the reamer doesn't have to cut as much metal and fewer chips. Seems like many gunsmiths don't have a pressure lube system to lube and push chips out continuously