Also, pull the bolt off the carrier, and dry fit the carrier with key to the upper receiver to make sure that the gas tube is indexed correctly with the gas tube.  If tube needs to be tweaked, should be done over the center of the barrel.

Hence may have a slight bolt bounce/gas tube not cleanly entering the key on lock up, and not allowing the bolt to fully lock up forward tight, with the carrier face tight against the back of the barrel extension. When this happens, back of bolt tail is not fully protruded out of the back of the carrier bolt tail channel, and the hammer will just shove the firing pin into the back of bolt channel, not the back of bolt tail itself.