Hard to see, but the rest of your brass looks like it's pretty close to having head separations. How many times have you reloaded them? Wonder if your headspacing is correct?
The brass was at the end of its life. I brought it out to that location with the intention that if any were lost, I wouldn't be upset. Usually I sit at a range with a brass catcher. They'd been reloaded 6 or so times. It was a PPU case that blew. I've had other brands last 8-10 loads.
Headspacing is fine on that rifle, granted it's not super-tight. It was set so that commercial ammo, along with any mil-surp ammo would feed without issue. I had a new locking shoulder made for it when I built it several years ago, and have checked it with go/no-go/field gauges and it checks out. My dies do tend to leave a shiny ring on the case at about the spot head separation occurs, so I've been running a paperclip down the inside walls to see if any are at that point, once the brass gets a few firings through them.
