I was thinking about it and realized I've gotten the same finish in tooling from a light oil fine lapping compound. I suspect it could be accomplished by sizing a bolt to the race then lapping it in with a lead lap set on a rod.

My above post could be an example of being very obvious but there's this concept of smithing that guns come together by magic. So if I was going to make a buttered Mauser, I'd first start with a well hardened receiver made from Swedish steel then would cycle through my box of bolts until I found a very close fit. Then I'd make a lap of either lead or jb weld on a mandrel, using rouge or rottenstone mixed with a light oil, then feed the lap in and out at a high rpm. I'd finish with a light 3m pad. Too shiny in the bore and it wouldn't hold oil.

In the end it's not a rifle, it's a tube of heat-treated lower carbon higher chrome steel with broached channels. That's how I'd make it slick, myself. The best bet would be to start with an under sized bore and lap it to size.