Note that "the strain is as great as calculated above" is NOT forward on the bbls but rearward on the breech. You do bring up some interesting points, but that the forward thrust on the barrel is not equal to the rearward thrust on the breech is really obvious, by the fact that when the two are bolted together the gun recoils "Backward".
A barrel would have some forward component of thrust on it from the friction of the load being pushed through it, but even in the case of the autoloader the breech bolt is locked to the barrel at the moment of firing thus actually exerting a larger rearward force to the barrel than its forward component.
