John,
Hard shot contributes to higher pressure. Before I noticed the 1500 PSI increase notice on the Nice Shot website I spied that my 7000 psi Nice Shot loads were slightly flattening the primers. That made me think... It wasn't long afterward that a fellow web-poster referred me to the Nice Shot warning.
I also recall recently reading something to this effect in an old shotgun book. I do remember that Askins wrote in The American Shotgun that 30% of the pellets are deformed upon firing. That book was published in 1921.
Mark
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