Split casemouths on brass hulls is often an indication of age. I go through several boxes of .22 WMR a year, in my Ruger 77/22WMR. I once used the remainder of a box of some old cartridges that I had used with no problem years before. The older ones split upon firing, almost every one of them. The only difference was the age of them. They had been kept cool and dry for several years.
I shoot vintage paper shot shells a good bit, too. Not unusual for case heads to split from rim to paper. I've never seen any sign of gas leakage from this, in shotshells.
SRH
