I picked up a 1907 Winchester in .25-35 today with 6 boxes of factory ammo, probably manufactured in the 60s. I loaded some up and ran about a dozen rounds to check for function with no issues. when picking up brass I noticed 5 casings had multiple splits After that I decided not to press my luck by firing anymore. What would cause this, ammo to old or what.

Thanks

The ammo was Winchester super speed, Remington Kleenbore, Peters high velocity