While thankfully I havent short stroked in a long long time, I have had students who do it often (not as often in the video).
I am wondering and perhaps more observing that everytime he SS it is with a double feed. I observed that his back stroke should have been more that enough to eject the fired shell. I could not tell if each time the fired shell was left in the chamber or if a new shel was chamberd but the gun "Double fed" the voice in the video says "double feed" after each incident. In both cases, empty in the chamber and feeding OR loaded round in the chamber still double feeding, I am inclined to think there is a mechanical problem as well as poor tecnique.
In my experience a "short stroke" leaves the chamber empty and no shell on the lifter (late presentation) or a fired rnd in the chamber and no shell called for (shell latch not released thus no shell called for) .
In other words,
If the empty is still in the chamber and the action was in fact worked far enough to permit a new shell to be on top of the lifter, that indicates a failure to extract and most likly cause by a weak extractor AND not pumping with enough force.
If a loaded shell is in the chamber and double feeds, then the action was moved far enough back to permit ejection (not short stroked)but a another shell followed the first one in . that seems to be a shell latch problem.
Opinions?
ETA: the fired shell does in fact eject each shot or (SS) and double feeds the next round both rounds above the elevator. Also "SS" occurs with two different shooters, plus the jamming at the end of the video suggest a stuck double feed shell one above the elevator and one below. I think the gun has a bad shell latch.