Motion compensating ship personnel transfer gangway - GIF

[Moby Duck]

Note that the you don't see the light plastic cup being placed on the barrel, and that the water is being blown away by the wind, and his shirt is being blown by the wind. An empty plastic cup like that would need to be glued to he barrel to prevent it blowing away. If not glued, the initial squirt of water would also have knocked it off. A better test would have been to fill the cup to the brim and see how much water is spilt. If the half filled water level surface is supposed to indicate that it is level, that is the nature of fluids, with or without stabilisation, the water surface will always be level. The shadow of the barrel on the deck shows that the gun has been trained slightly aft, (or the ship turned), but there is little evidence of any relative movement in roll between the fixed structure and barrel, or of the ship actually rolling. When you look at a stabilised barrel it should appear to be moving, relative to the ship. If they want me to believe this, they need to film it in poorer sea conditions, slew and elevate the gun, show me the cup being placed on the barrel without glue, and fire a burst from the gun. My opinion is that this is a fake video, the gun is probably not even turned on.
Something may have been lost in the translation but 30mm guns can hardly be classified as "artillery".