In doing these BB fired tests did they by chance conduct any with the barrel secured from recoiling with the shell & if so what occurred. If the barrell recoiled with the shell this simply means the inertial resistance of the barrel against moving was less than the frictional resistance of the shell to chamber. This "Most Certainly" does not ensure that if the barrell were given more resistance against moving the shell would remain firmly affixed in the chamber.
In support of the fact there is indeed back thrust to the shell head is the number of cracked double frames which have occurred over the years as well as certain Pumps which have "Earned" a bad reputation due to a weak bolting design.
While not shotshells no reasonable person can deny that the self loading action of the .22LR, .25ACP, .32ACP, .380ACP, 9mm Makarov, a few 9mm Parabellums & .45ACP sub machine guns etc is brought about purely & entirely from the back thrust of the cartridge.
