Good luck with the "Black Sea" gun. I suggest you shoot it with both hands on the rear grip at first as there is some recoil if you use all the power. I have never been brave enough to try shooting my gun with one hand as the rear hydropump pivot bar is stainless steel and I would not want it hitting me in the face, but on your gun this metal pivot bar has been eliminated. After some familiarization shots to get my aim correct (not hard as the spear flies virtually horizontally in a straight line from the gun for about 6 metres) I killed everything that I aimed the gun at which could be carefully lined up, which was an outstanding result for a new gun on its first outing.
It is worth making yourself a short piece of aluminum tube to free the shaft stop ring if it jams, you put the tube over the shaft tail and hit it with the butt of your dive knife and the stop ring will come free as long as you brace the bare threaded spear tip against a hard surface, like a rock (remove the actual spear tip of course). The stop ring going from zero velocity to 45 metres per second (or more) in an instant it tends to be hammered by the shaft tail. By reducing the number of hydropump strokes the gun can be used to shoot smaller specimens at closer range, I use either half the number of strokes or the full complement. Too much power variation and you don't remember how the gun shoots, so stick to a few options with a known set of strokes. Multi-power guns are confusing to use, so selecting two or maybe three options is all that you need. Watch out for other divers, I never use my "Black Sea" gun without checking the whereabouts of other divers as you cock the gun with it laying across your knee while you are working the hydropump lever. If the gun discharges unexpectedly (it should not, but it is better to be safe than sorry) then the shaft will fly off to one side where you would not normally be looking and any resulting accidents will be big ones.
The "Black Sea" gun has a trigger safety of sorts, but don't be tempted into trusting it. I have never used it on my gun as I wrap the shooting line before I charge the gun and the safety is therefore not required as I am in full control of the gun, from the rear end, once charging is completed. If in doubt and a risky situation is developing then just pull the spear free from the charged gun and execute a dummy shot to release the pent up water from the gun. Once out of the gun the spear will not go in again unless this is done.