Let's look at what we know &what we can with reasonable safety assume.
We know that we have two tubes, one fitting inside the other. We know that one tube is open on both ends (the Barrel).
i think it sfe to assume the other tube is sealed solidly on the rear end with a fixed firing pin in the center.
thus when the inner tube is pulled forcibly enough to the rear to ignite the primer the gun fires.
now the solid end of the outer tube is fixed solidly to the stock which is supported by the firs shoulder. Thus the hull is not going to "Jump" out he back of the barrel as it is solidly supported from the rear.
.the real question then is why doesn't the barrel move forward off the hull from friction of the load going down the barrel. I think the answer here is the firer has just pulled it forcibly to the rear & still has a firm grip on it. Also as the barrel is large enough to accept the head of the shell it does not have a tight fit with the wadding. No doubt there is blow by & the load does not acquire full velocity. With the combination of these factors I think there is simply not enough force to blow the barrel forward. I do not see this in any way disproving that a shell has "Back Thrust" when fired. Obviously some classes of shells do have back thrust as .22RF, .25, .32 .380 ACP, 9mm Mak, 9mm Parabellum & .45ACP have all been used successfully in "Blow Back" designs. If there were no "Back Thrust" a blow back simply would not operate.
I do not know of anyone either repealing or amending the law that for every action there is an equal & opposite reaction.
Anyone who thinks there is no back thrust involved simply find you an old 870 Rem, remove the locking lug from the bolt, reassemble it, load her up & "Fire At Will". Prove the rest of the World Wrong, IF You Dare.
