Split case, you take a dremel tool and cut the neck, and a tad bit of shoulders down teh case. Now you can bend the neck to hold a bullet. Not too tight, but tight enough to move the bullet. Insert a bullet very long, chamber, and carefully remove case and measure COL. Do this several times.
i do about the same thing. First remove the firing pin and ****ing piece from the bolt. Insert a neck sized cartridge with the bullet way long. Lightly grease the bullet to keep it from sticking. Chamber and remove the cartridge carefully. Then measure the OAL of the round. Set your bullet seater to make a round that is .015 shorter than the OAL of your dummy round.
Problem is that rounds set up this way will not go into the magazine of some rifles.