I've seen that with 17 HMR. With the round head spacing on the rim, is the chamber shoulder not too far forward? It wouldn't take much clearance to allow the neck to crack with hard brass. Just wondering. That HMR ammo was not old and besides, shelf life for ammo to crack is BS. I've had that with old 303 Brit ammo but we are talking pre WWII ammo. Mind you, I've had cracked necks in my 25/303 that were annealed less than 20 years ago that were reloaded once or twice and not loaded since.