Immediately after firing the pressure dislodges back the primer from it's pocket and slams it hard against the still protruding firing pin, that's how it gets pierced. As the pressure builds up the case also comes back against the breech face relodging the primer in it's pocket.
Edit. The descripted above happens in all situations, but with proper headspace the primer backtravel is kept to a minimum, not enough to piece the primer.
Last edited by kurusu; 06-15-2017 at 05:42 PM.
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