Well certainly there is some amount of friction holding the case in the barrel. I'm afraid this test doesn't really prove a whole lot though. Some force is obviously going to be lost when the primer blows & pressure is vented out. A battery cup shotshell primer leaves a rather large vent hole. The BB gun was set so close that an indent of its muzzle was imprinted on the shell's base. Who can say if it had not been there if the shell would have been totally blown from the gun or not. As soon as the primer blew pressure fell & burning was not complete. Also what would have occurred had the barrel been more firmly fixed rather than being able to rock so easily on that rather small stick of wood.
I do recall guns having fluted chambers, but don't recall they were the "First Blowbacks". Several companies were making pocket auto pistols on the blowback principal more than a century ago & I don't recall any of them having other than a standard chamber & the chamber did not need lubrication for them to work. The higher pressured 9mm largo was made to work as a blowback design in the Astra 400 by use of a very heavy bolt & strong recoil spring, so obviously a good bit of back thrust there. The .45ACP worked by blowback in the "Tommy gun" also by use of a heavy bolt.
I also recall rifle pressures being measured by measuring back thrust while using oiled cases to cut the friction. Apparently they gave reasonable accuracy, though I am certain they would give a different reading than a modern Piezoelectric sensor.
