Home Forums Hunting & Shooting Ask The Gunwriters Old load and neck splits?

I recently found a pop bottle crate of assorted loads and brass of different calibers stored in the garage and forgotten about since moving here in 2016. Among them were several .223 Rem rounds, some partial boxes of factory loads from probably 20 years ago and some reloads anywhere from probably 2005 to 2015. I decided to shoot them up more to empty the brass than anything else. With the factory loads the brass came out fine. The only reload I tried was my old standard load of a Sierra 40 grn H.P. with 28.0 grains H335 and a gm205m primer for 3600 fps in Winchester brass, not by any means a hot load and it always gave good brass life. Any notes of how many times the brass has been fired were lost in the flood before the move. The ammo wasn't touched by the flood. There were about 30 loaded rounds and the rest of the 50 round box had empty brass from the same lot. I fired two 3 round groups and when I went to pick up the brass I noticed that 4 of the six rounds had split necks. I examined the other previously fired rounds and the neck appeared fine. Could age have hardened the brass to cause the split necks?

Thanks.