Pneumatic dismantling: how to get inside

The real pressure seal on the piston is the rear "O" ring, although the cup seal behind it can also hold pressure as I had a gun with the "O" ring damaged and the gun still held air. The front "O" ring on the piston is more of a guide and it is that which you can feel hitting the uneven surface where the rust has stuck which transferred from the piston's corroded rear metal section. Some pistons don't have two "O" rings, they just have the rear one along with the cup seal. If in doubt replace the piston as once you get rust on an item it is more susceptible to rusting there again due to the rough surface where rust once was. Also guns need to be thoroughly rinsed after use, especially vacuum barrel models due to the water trapping mentioned before and that includes pushing the piston back with the then exposed surface being flooded with freshwater and then being let out. A gadget can be made to replace the spear in pushing the spear back a short distance, something like this one from Nemrod, however it needs to be of the shaft diameter or slightly less where it penetrates the muzzle.

I am actually surprised that after all this time manufacturers of vacuum barrel guns or kits don’t offer such a piston pusher as a standard accessory. The advantage besides being short is they don’t jam in the piston nose when you push the piston back, so are easy to pull out of the gun when you finish pushing.

These pushers could be made of Delrin or one of the harder plastics similar to that used in piston bodies and shock absorber anvils.

A diagram showing how saltwater traps. Note even vacuum barrel guns have water enter the inner barrel as it was in the muzzle entrance when you loaded the gun underwater with the insertion of the spear tail.

A handy thing to have is a drift or punch set to knock pins out. Sometimes the pins will come out very easily, but if in place for a long time then they may be rather stubborn to move. A set of steel punches like these are just the thing as they should centre on the pin rather than skid off which you don't want to happen. Practice makes perfect so try it a few times on something that you won't worry about rather than your prized underwater weapon. A drop of oil added earlier and then wiped off may help as the lubricant may wick down the gap between the pin and the bore hole that it is sitting in. Don't use a big hammer and try gentle taps at first as usually once pins begin to move they will easily push out by hand.