Greetings 308mate,
My impression is that some small rifle primers crater at lower pressures than others. Cratering is a step on the way to piercing. You could try magnum primers or other primer manufacturers or drop your load until the problem goes away. The question needs to be asked is why would you use small rifle primers in the .260? I know they can give a small accuracy benefit in bench rest rifles but they can also result in hang or misfires in cold conditions. Perhaps it would be best to stick with large rifle primers and eliminate any possible problems.
Regards Grandpamac.
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