Core Bits

I'm looking for a source for decent core bits for position marker shell dot making. I would like to be able to make dots of about 5.5mm. I have made 1/4" dots with a regular plug cutter but they are a little too large for me. The diamond core bits I see always give the outside dimension but rarely if ever give the core size. Anyone know of a source?

Michael, what we have done on occasion is cut off a piece of fine metal tube (brass or steel) and then use a key file to cut tines into one end, which we then sharpen with a small diamond file. The tubes usually give you an exact measurement for the inside diameter, so something like 6.0 outside/5.2 inside diameter.

Hope this helps.