Thanks Pastera - I get the idea... heavier bullet taking nano-seconds longer to move into the rifling = higher pressure inside a case with less room to expand.
seems reasonable - this is why I went with 6.6 and (not 6.8 gr.) and the (4 gr. cut 121 gr. from the 125 gr. manual) and lead soft Berry's material...
I will capture one round under a towel and eval the primer and other case evidence before accepting this load as safe... and in the future start at starting loads with new untested bullets.
much obliged -