There are lots of reasons for primer pockets to loosen up , excess pressure is just one , The batch of brass used to form the cases is another . The metal itself has a lot to do with how well it will hold up , recycled brass melted down will be different than say Virgin brass due to the compounds used .
I have had Lapua cases that have worn out after 3 loadings , yet had PMC brass that has been loaded over 10 times , the round itself can also cause problems due to the pressure that it operates at , a cartridge that runs at say 18,000 PSI should hold up much longer than say a round that runs 50,000 PSI .
The number of times a case is primed will also make a difference , as will how the primer is installed .
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