Dimner
I do believe in Captain Crunch.
Id just use a coarse sharpening stone. As long as it is not a waterstone, it will be flat.I'm currently waiting for the cheapest shell holder I could find to be delivered. I'm gonna try the idea that NorCalMikie mentioned, using a drill press and sandpaper/stone. I've done them very crudely before on the side of a grinding wheel, and although it worked, the face of the shell holder wasn't square. I think without a lathe or mill, the drill press is my best bet. I agree with you guys, I'm surprised someone doesn't offer minus shell holders! (Somebody wanna get on this? Lol)
Also, an easy method would be using spray adhesive and a piece of 150 or 220 grit sand paper on a flat surface. Plate glass or a spare piece of thick flooring tile, or the cast iron table of a table saw or some other tool. You can check your progress by using a black sharpie on the metal that is being sanded off. Use goo gone to clean up the spray adhesive.