Mark IV,
I suggest that you just wait until your barrel arrives. With the 300-221 Fireball, when you form your brass you can set the shoulder exactly to match your chamber so the headspace is perfect. If you want to get a jump on making brass, I would just trim it to length (~1.400) and not form the shoulder. It you look just like a rimless 357 mag cartridge.
(See my post here:
http://www.quarterbore.net/forums/showthread.php?t=1763)
Once your barrel arrives and you mount it in the receiver, you can then start forming the shoulder. Mount the resizing die in your press. With the ram all the way up screw in the resizing die until it just touches the shell holder and then back it out about 0.02" to 0.03". Now make a singe piece of brass and try it in your rifle by putting it in the chamber and gently closing the bolt. It should not fit because the shoulder is too far forward. Do not force it or else the case will get stuck in the chamber and you will have to use a rod to get it out. Now screw down the resizing die a bit and try it again until the bolt just closes with a slight effort. You can check this by coloring the shoulder with a sharpie pen... You should see a ring on the shoulder where is contacts the shoulder of the chamber.
Now you are really close but slightly too long. If you are using a bolt rifle you can run with the shoulder about 0.002" to 0.005" long. When you close the bolt the cartridge will be press into the shoulder of the chamber and will be tightly supported. This should give you very good accuracy because the each cartridge is "fit" to the rifle chamber for the best consistency. If you are shooting a semi-auto you do not want a really tight fitting headspace because it can have an adverse effect on the cycling of the action.
Hope this helps and good luck
320pf
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