O K
First, I have reloaded for about 25 years and in the thousands of rounds reloaded, have had only one other primer rupture and that was on a 22-250 many years ago when I was reloading 250 rounds a week for varminiting.
I brought 270 and 280 rounds to test in two rifles, a 270 Colt Light Rifle and 280 Browning A-Bolt. I had loaded five rounds each of different loads to test for groups. The first shot of group A for the 280 Browning popped the primer. I later at home checked the weight of the powder in group A and found it 10 grains over, I believe that the powder was IMR 4895, and remember that I said that I usually load light.
Shaken I reached over and grabbed load B, but by error (NOT MY DAY) I pulled group B of the 270 loads. What supprised me was that the primers backed out, indicating that the case was not against the bolt, but the case did not rupture or split. I had actually thought that I had set the bolt back somehow when the primer let go and that was why the primers were backing out.
I hope that this clears things up.
Anyhow, it's all my wife's fault because she wanted a 280 instead of a 270 like mine.
C F
(last statement was a bad attempt at humer)