Photos below are of a deburring tool and deburring tool base, both made by RCBS, and likely used by everybody here. As we all know, the left side of the tool cuts an outside chamfer on the neck of the brass cases for us. It would seem to me that it could be modified (or a similar tool made) to cut a gas check groove on a plain base bullet. Not something we'd want to do hundreds of times a day, but still possible to put a gas check on a few dozen plain bases bullets for testing or whatever. What do you machinists think?