Step 2: Starting Construction
2) Use a thick piece of steel and an angle grinder to but a jig the same size as a USB. This is sort of a guess and check. I used a sharpie and a caliper to get close. Also chamfer the top edges so you can get the brass started. Be sure to make each chamfer identical otherwise you risk stretching out or tearing one side.
3) Stretch the cartridge over the jig gently with a hammer. I used several very light strokes to guide the brass onto the jig.
4) Follow point 2, grind a second Jig that fits the smaller rifle cartridge.
5) Stretch the larger cartridge gently over the new jig just like in point 3.
6) See how they fit.