Slam fires can and do happen in the AR rifles. Ever see a round you chambered then ejected. Well, LEO's are chambering the same round over and over. Each time that firing pin hits the primer. Till one day it lites it off. It happens a lot more than is told as noone wants to be that guy that shoots a hole through something so it is not reported unless it is seen by someone else.

CCI 400 primers are not made for loads over 45K psi. They are made for the 30 carbine and 22 hornet. You can use them and get away with it if your parts are not made over sized(rem 700) and you don't load 556 pressures