What sort of dies are you using Jas? Can you remove the sizing collar off the stem but retain the de capping pin? If so get some cases, decapp them and fit new primers with no lube anywhere. Fire them off and if they all go, you can start to look at say a lube/primer issue. If not you may have a bad batch or a sizing issue.
Just got to narrow it down bit by bit as it is hard without seeing things first hand.
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