RE: Blew a primer!!!!!
You say you have shot this IDENTICAL load before, and had no problems....You changed nothing at all?
What air temp. was your load worked up at, and what was the temp/humidity when you shot it with no problems compared to the temp/humidity when you had the pressure problem?
Some powders are temp. sensitive and can cause problems at or near max loadings if developed in cold weather and then fired in much warmer weather.