Drill a hole in the bottom of the case, use the tap to thread it, and you should be good to go. You might need to put a little crimp in the fired case mouth to give a little tension to hold the bullet. You should also make sure the the fired cases aren't too big to go back in the chamber before you drill them.
-J.
You definitely do NOT want to crimp the case. The whole point of the comparator is that the bullet slides easily back and forth and doesn't stick.
You can use a fired, unsized case if it slides easily into the chamber and the bullet slides easily in the neck. You'd be better off if you had a Redding body die, so you could size the body and bump the shoulder back .001 but maintain the loose neck.
Drilling the case and tapping it should be fairly simple. It's probably easier to hold while drilling if you have a small strap wrench. Might be tough to hold with your fingers.
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Another method for determining max OAL is to get a wooden dowel small enough to fit down the barrel and longer than the barrel, most any hardware store has them at 48" for under $1. Cut one piece off long enough to reach into the chamber from the bolt end.
Close the bolt and slide the square end of the dowel down the muzzle until it touches the breach face. Use a razor blade and mark the dowel right at the muzzle.
The second part is a lot easier if you have help...
Tape a bullet to the end of the cut piece. Slide it GENTLY into the chamber until it GENTLY touches the rifling. Slide the same end of the dowel as used in step one down the muzzle again until it touches the bullet. Be careful to make sure it's touching but not pushing the bullet away from the rifling. It's not hard to do. Have your helper mark the dowel at the muzzle with razor blade.
The distance between the marks is the max OAL with that bullet.
