That shiny ring is called "INCIPIENT SEPARATION" IF THIS HAPPENS ON THE FIRST FIRING OF NEW BRASS OR FACTORY AMMO, YOU PROBABLY HAVE A HEAD SPACE PROBLEM. If the brass is old it is time to get new stuff. This can happen on brass that has been over loaded the first shot, but normally takes a few reloads to show up. Check your chamber to be certain there is no problem with it.Make sure you haven't "set back" the bolt lugs,if this is a bolt gun. A local gunsmith should have a GO/NO GO gauge for the head space. has accuracy changed since this started?