This is kinda embarrassing because I have a feeling it’s gonna turn out to be operator error somehow. When fire forming 300 WSM Norma brass in a 7MMWSM chamber, approx 1 out of 5 primers showed extreme flattening but no distorted firing pin dents usually associated w pressure and no hard bolt lift, etc. those cases also stretched in oal drastically. They were all .007 to .010 under after necking down but the worst bad one was .009 over after firing. The rest showed no flattening at all. I’ve fire formed hundreds of cases in that chamber using the IDENTICAL procedure, min. Load, brass, bullets, etc. and never had problems. ????I’m guessing that when the chamber warmed a little the cases were looser and could bounce forward, letting the primer come partially out then "hammering" it when it bounced back???? This doesn’t explain stretching the OAL though.