How to cut it

If the bar is short it could be done with a rotary table and say a 2mm milling cutter which would allow you to adjust groove width.

If in the lathe then grind up a tool that looks a bit like a parting tool but has a lot of side clearance, often a half round D bit will be the easiest shape to make and will have plenty of side clearance.

Isn’t that what JasonB’s suggestions are about?

Have you seen annular hole cutters from Rotabroach for example? Since you are cutting brass you should be able to produce a similar thing out of steel to fit in a tailstock chuck and cut your groove. Does not have to do a lot of cutting so I would try it unhardened and see how it works on a spare piece of brass.

Martin C

Oops sorry about that Ps3, I have occasion to cut annular grooves (50 or so at a time) and made tool for it,which seems to work quite well. It has relief on the outside/inside and the cutting edge.