I want to try lathe boring a resizing die from screw stock. I need to know how much smaller the die body has to be to reliably size a case small enough to chamber. I have read that roughing reamers are .002" undersize and can be used to make size dies. Is .002" really enough to account for brass springback? For example, for a 280 Ackley chamber with a shoulder diameter of .455" I would need a size die with a .453" shoulder?

If you are doing a bottle neck case, just neck size it only, don't do the shoulder.

On straight wall casee, I insert 5/8" drill rod into the threaded stock. It is easier to hold and bore the .625 drill rod. Easier to change the taper, in case of a miss calculation. Just make a new insert.

also:

The neck size should be .006 [min] under size, the body could be .004.

You can also make it in segments. Neck and shoulder, then another for the body taper. If you have access to a ID-OD grinder, you can harden an then grind them on center for perfect alinement.

My only experience with this was making 20 tactical size dies, which resize a 223 down to 204. I found .003 was not enough and had to go with .0035 to get adequate neck tension.