When I quit competing in NRA across-the-course High Power rifle matches almost 20 years ago, I had several hundred loaded cartridges of various loads and just left them in my reloading shed. All 308, mostly loaded with 168 Sierra BTHP, 40.5 gr of 4895, Win primers, Federal brass. Bolt gun, rapid fire ammo. All twice reloaded, full length re-sized in between loadings. All always shot in the same bolt gun. The gun's chamber measures "0" and I was resizeing to "-.002".
I decided to put my target scope on the bolt gun and do some informal target shooting. While at the range and in the process of sighting in the scope I shot seven rounds without looking at the fired brass, only concentrating on the target and scope. As I took the eighth round out I happened to glance at it and the complete neck was gone. I then examined the preceding seven and on each the neck had split lengthwise all the way from the mouth to the shoulder, some had two splits. Some had also split horizontally partially around the neck/shoulder juncture. Luckily when I raised the muzzle of the rifle the missing case neck fell out of the chamber.
I have loaded and fired many thousand rounds of this same load combination and never has so much as a misfire. The only variable I can think of is the ammo being stored in an uncontrolled environment for almost 20 years.
Has anyone had a similar experience or any thoughts on what might have caused this?