Guffey, SAMMI does list head space for the .300 Win Mag case. They do for all rifle cases. Check the following SAAMI info noting the red-circled dimensions for what's commonly called "case headspace." Note the .300's got two of them; one for the case belt headspace and one for the case shoulder headspace. Case shoulder headspace is the one most often meant by the term.
There are gauges made to measure case headspace by Hornady and RCBS.
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Last edited by Bart B.; March 4, 2013 at 02:54 PM.