In the first video for all except the last shot he had the breech against a tree, so obviously it wasn't going anywhere. Without data as to the actual weight of the two breech sections its impossible to determine why one Flew Away & the other didn't. The one which did makes it obvious there is indeed casehead thrust.
On the blow forward action the case head was supported by a solid breech with the bullet entering the barrel pushing it forward.
As the gun was able to operate & the extractor stipped the case free it would seem this conclusively proves the case adhesion was not equal to the force applied to it. Had it been it would have simply acted as a single shot.
