Originally Posted by
dirtball
Was this the last round in the string, or in the middle of the string? If it was the last round I would look at the firing pin spring. If it was in the middle of the string and none of the others looked like this it was probably a bad primer. But I would still look at the flash holes in all of the cases for variations, a large flash hole may have allowed a bunch of powder to get into the primer pocket, or a really small flash hole may have allowed the pressure from the primer to cause this. In any case I would mark that case and see if it reoccurs.
Dave