[velocette]

Flintlock50;
Your brass has been worked too much, that is stretched. This can be from several causes. One might be you are pushing the shoulder back too far when you re-size. Another could be your rifles headspace or chamber is on the large size.
For the above problems, if yours is a modern bolt action rifle, you might look into just neck sizing your brass.
If your rifle is a gas gun, (M1, M1a etc.) or many military rifles, 3 ~ 4 reloadings are about all the life you are gonna get from your brass. This is due to the large chambers these rifles have to assure proper function when dirty and in some cases stretchy actions.
One thing for sure, your brass is no longer suitable for reloading. If one or two out of a lot are cracking, you should discard the lot, assuming they all have been reloaded the same amount or even close. (a face full of hot brass particle and powder gasses is never a good way to end your days shooting.)
A piece of stiff wire with an ' L ' bent on its end can be run into your cases to feel for a thin area (like a channel) near the head (right where your cases cracked) as another way of testing cases.


Roger