When your done let us know how you held the case while you tapped it. It took me about 4 cases to get it right. I want to see what you come up with. Also when drilling brass you want to modify the drill bit. A standard drill has a positive rake on the leading edge. This helps pull the bit into the steel. This will not work for brass. You will need to grind or hone it to where the angle is neutral so that it peels the chips up but does not pull into the material. In the shop we have grinding wheels that we keep the corners square for you a small hone to just break the edge may do the trick. There is probably a YouTube video demonstrating this.