So, degas the gun, insert your pull-ball keeping the long twine out, then take the scotch-brite and remove ALL the burrs and hard edges on the air-tank's threads. Once you have done it, clean the thread with some of the pellet tin sponges and some alcohol, run your finger around and make sure there are no sharp edges.
so mods, like completely rebuilding the trigger assembly so you can adjust trigger travel to 1.5mm and all the slop is gone, with new pins, shims and a grub screw added to the sear for a trigger stop. or maybe using a Stormrider barrel cut down to 13 inches and using 12mm ID to 15 mm OD carbon fiber tubing so you can add the Air Venturi muzzle ...