My thinking is only that if the steel shot does not compress within the shot cup it still, by friction, wears against the bore. The plastic cup protects the bore from steel rubbing against steel but not from the friction of the steel not compressing. It simply wears on the bore gradually, diminishing whatever choke is in the barrel if it is a tight choke. That is the best this old guy can explain it. Darn it, now you have me thinking about it more and more. Remember I really don't know anything about this so this is only a thought on my part. Is there a real ballistician in the crowd??? Anyway that is my thought on it.
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