The rifle may headspace fine on go/no go guages but what we are saying is when you run your brass through your full length sizing die it could be bumping the shoulder too far back and therefore creating too much headspace. If you are not directly measuring how much the die is moving the shoulder back with a guage that could be the problem.
If you've made the case too short it will slam back into the bolt face and you will get a very flat primer.
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