223 split necks

I personally think the "heat cycles" such as seasonal storage, in a non-controlled temp. warehouse, can play a big factor. But then again, it may be a production annealing defect or something with the brass itself.

Back in the 1980's we were buying cases of brass, Berdan primed, .303 British, that was loaded with Cordite and Cupro-nickel jacketed bullets. It was produced in the middle east sometime around 1900. Navy Arms was selling a big cashe. It shot fantastic, with an orange'ish red fireball. I don't remember a single misfire. No two pieces of brass were the same length either. They charged the cases with sticks of Cordite, added a card over paper disc, then necked the case with a bullet in place. Crazy. I figured it was produced, delivered, and never moved until they found it and sold it some 85 years later. A consistent climate and good storage.

Then as you have. Some ammo perhaps not 10 yrs. old with several split necks! What did it? Obviously it is shrinking somehow and the brass molecules can't stretch that much, so they split. Age, expansion cycles, too hard brass? Who knows. Salvage the pieces, cut off the brass to reform into something else. Those bullets would shoot fine in something else. Waste not want not.