Build a jig by drilling a hole (9/16" or what holds tight without deforming the brass) in a 1x4 or 1/2" plywood board. Recess the bottom with a router for the case rim -- but not too much so the case protrudes just a bit so it is tight when clamped onto the drill press table. I think I will try it first without recessing the bottom -- If it rocks too much, then I will recess it. Make sure the case is centered on the grinder bit. Set the stop on the drill press to cartridge length I want. Then grind away. If I need to support the case walls while grinding, I will insert a section of 1/2" hardwood dowel that is below the top of the case. When done, I will put the cases in the vibratory cleaner to take any minor burrs off the brass.