You are doing great Rocketman, hang in there.
Too bad we don't have a stackup drawing to picturize all the roll centers, clearances, and the directions of the forces being applied.
It could be called, "Why a shotgun shoots loose", and be an FAQ stickey.
The timing of these controlled explosions (I know, oxymoron) moves through the shotgun like a wave. Each part accelerating like little dominoes. Heavy pieces being more difficult to accelerate, and taking a longer time of collision. Like catching a baseball. Human bucking up all the while, squashing the stock into the action.
Unfortunately, by the time the stock is accelerated, the chambers have been at,exceeded, and vented themselves due to the pressure.
So, I vote for excess pressure doing more damage first. Not to say, that as links in a chain, little weak parts along the collision highway could fail or be beaten to bits before the stockhead pulped. And be done below barrel burst pressure, or fatigue cycles entered the mix.
