Greetings @Hugh Shields,
In addition to what @Oldbloke has said above I suspect that your Super cases are quite old. You could try loading a small sample and firing them to see if you get neck cracks on the first firing. If you do you might want to fire the primers and anneal the cases before your first load. Close to 50 years ago I had problems with neck cracks in Hornet cases in what I think were Winchester Super cases. You also might want to do some check weighing to help with your batching as Hornet cases have varied with weight over time.
Regards Grandpamac.
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