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Two best ways. First the Bestest Easiest Way. Shop around for some good "TAPERED" feeler Gages. Of course, the tapered gage MUST fit between the extractor and mortice side wall. Insert the blade with the action slightly open, then close the action, trapping the gage between the bolt face and the cartridge base. I suggest you also use a de-primed case. If your chosen blade will not allow the action to close ..................
Second best way. With a standard constant width Feeler gage set, clamp the blades tightly closed. With your terrible and dangerous Dremel Tool, grind away one side until you have a reduced with blade that will slide past the extractor. You may have to lightly dress the side you have ground.
Optimum Head Space is .004 at +/- .001 Some rifles will still function say out to some really ridiculous head space however I recommend it be corrected. In most instances, you will find too much head space in large bore rifles and too tight in small bore guns. Fixing the head space is the really fun part.
Coffinmaker