I was doing load development 7mmRM in a 700 Remington. I was getting at the top end of the load. Had shot 2 rounds of this load, had primers getting flat and bit of crater with a shiny spot on the brass base once in a whle. No shiny ring on the side of the brass. The third shot stuck the case. Had to knock the bolt handle up with my palm the last half inch, then it wouldn't extract. I hit the bolt with the heel of my hand a couple times and the bolt pulled from the rim. I checked the bolt and it looked fine. The primer in the case inside the chamber look very well also, flat primer with some crater. When i got home from the range i took a cleaning rod and inserted it down into the brass and tapped it and the bottom part of the brass came no trouble, just where they usually separate. I thought i would have a hard time getting the rest out, but just running down the cleaning rod again with a slotted end in to catch the case neck and the brass dropped right out. What i'm stumpped with is the case had a huge dent from below the shoulder to the edge of the case neck. What could have caused this. I sent a lot of rounds down in nearly 40 years and never saw this before.