G'day,
This popped into my head randomly so don't have a picture of a particular example, but I expect you'll know what I'm talking about...
I think the term is a 'machine finished' bolt for rifles where the cylinder of the bolt has a pattern machined into it. Typically something like a spiral down the length of the bolt.
Make sense? Know the one?
Does that do anything in terms of working the bolt for a smoother action etc?
Or is it purely cosmetic?
Cheers.