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Perfect for slam firing.Looks like one of those new round head primers .
I do not have the brass in hand. Sent to my by a friend.
My guess is is was mixed into the 500 smith brass and when the previous owner tried to decap his 500 brass he got a surprise of a 40 case jammed inside. No where for the primer to go really. Pushed the decapping pin through the primer.
I don’t think overpressure could do this with a breech in the way. If it were fire when the action was out of battery you would see a lot more weird stuff going on with the case.
JC how big is the flash jole in the 500!The 40 case was inside a 500 case and someone decapped both.