Greetings @tikka,
I had not heard of the Sheridan gauges before so looked them up. They only check if your cases match factory specs and do nothing to make sure they are not too loose in the chamber. 1/8th of a turn of the die works out to 0.227mm or about 0.009" so I certainly hope that they would not chamber. There are four 308 rifles in my cupboard and all have different chamber lengths and I neck sized for years until I broke open the piggy bank and got a Hornady comparitor kit and a Redding Competition die set so am able to ensure that all cases are snug in their chamber. The only reason I know for a head separation that I know is excessive head space from either the rifle or the dies. This is where you need to look for the problem. The comparitor kits are not excessively expensive and measuring new, fired and resized cases should show the problem to you.
Regards Grandpamac.
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