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You need a set of head space gauges. Brownells or Midway sells them. If you google the term head space you well find a lot of info on how to do it and what you are measuring. Basically it's the distance from the bolt face to the back of the cartridge. To much space and the cartridge can rupture, to little and the bolt does not close.
On the M1A you are looking for a specific number that well allow you to use both 7.62 and .308.
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