The 300/221 / 300 Whisper is a wildcat, and like any wildcat, dimensions are slightly "flexible".
Get a chamber cast done if possible.
Make up a few cases with bullets you plant to use, and measure the neck diameter. Have the 'smith that reams the chamber measure the neck clearance, make sure its adequate for the bullets you're planning on using. get the dimension if possible.
I had the 'smith that reamed my chamber ream a piece of left over barrel just past the end of the shoulder, that way I'm not trying to figure out the seating depth on the rifle, but with a piece of barrel thats 2" long and easily handled at the reloading bench.
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