There is a formula. The case neck thickness x 2 plus .264 -.002. You want to go .002 under your loaded dimension (.001 spring back and .001 tension is common) You probably have a no turn neck(.293?) in the chamber. You will have to measure your brass but I'm thinking close to .288 bushing. Lone Wolf or long range can tell you what Lapua 6.5 neck walls measure. Wait for more input or google it. i just thought I would throw some common info out there to get your wheels turning.
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