Here is an idea for cleaning residual saltwater out of pneumo-vacuum guns which have fixed muzzle seals. The bottle-shaped gadget is used as a loading plunger with the long stem section replacing the spear and ideally being made from hard plastic. The place where the cross-bore drilling is located could be slightly thinner in diameter in order to stop the edges of those cross-bore holes damaging the muzzle seal. The cleaning operation would be as follows; you partially fill the bottle with clean water and then upend the device and push it down through the muzzle seal. As the piston is pushed back down the gun the vacuum developing in the inner barrel sucks the water out of the bottle, but if you let the bottle slowly come up then the hydrostatic pressure in the barrel pushes the water back into the bottle. A few up and down motions of the plunger will clean out the inner barrel and around the front end of the piston replacing any saltwater film with freshwater. If the muzzle seal leaks water when pushing outwards from the interior of the gun then the water is lost, but the cleaning action will still be the same. Also if you let go of the bottle then the water will push the stem out, so you need to keep a good grip on it, but the stroke will be small, so not likely to be dangerous in terms of the bottle flying out of the gun.