Could be excessive headspace, but other possible causes too.
NRA Shooting Illustrated: Problems with PrimersDave Emary, Hornady's chief ballistics scientist, says excessive headspace can cause primers to back out or even pop out completely. There can be other culprits, like a rifle chamber cut very tightly, a throat cut too short or a barrel with a tight bore. On rare occasions, the cartridge brass might not have been properly annealed. Temperature can also be an issue, and this was my suspicion.
Headspace is like Goldilocks porridge. It has to be just right for the best accuracy. Too much is bad and too little is also bad. So how do we know when we have too much headspace? There are a couple of obvious tell tale signs on our fired cases that are sure fire indicators. The first is a protruding primer. If, after firing, you see the primer is backed out of the primer pocket, even a little bit, you have excessive headspace. Remember, the top of the primer in an unfired case is supposed to be located just below the case head after it has been seated. If it's sticking out after the shot has been fired, the case is too loose in the chamber.
The second indication of too much headspace is a excessively stretched case. When the powder is ignited, the case is expanding in all directions, including front and rear. Brass does have a limited amount of "springback", but when the amount of headspace exceeds the ability of the case to springback, the case will be permanently stretched and weakened. Stretched cases can be easily identified by a bright, light colored ring located just forward of the head. The case at that point has been stretched dangerously thin. Those cases should be thrown away immediately as they will come apart at that point sooner or later. Case head separations can sometimes be very nasty.
