I thought that I would complete the list of pneumatic speargun types by describing a speargun which has no inner barrel and in a sense no air pressure tank either. This is the vacuum speargun which is powered by a relative vacuum formed inside the gun body when the gun is muzzle loaded. The gun has a large diameter body tube (internal diameter is 40 mm) to provide the cross-sectional area necessary to obtain some useful propulsive force from the pressure differential acting on the sliding piston that will propel the spear from the gun. The further down in the water column you go then the power of the gun increases as basically with the rear end of the gun tube open the gun shoots via a controlled implosion.
https://youtu.be/A5Q37RnB-w0 The gun shown here is another version which seems to have an even thicker spear than the original which is being described in this thread. It also has a mighty big trident!
The spear (8 mm diameter) inserts through a muzzle seal and drives a large diameter piston back (same size as the inner diameter of the body tube and carrying a single large diameter rubber seal), pushing it away from the muzzle and moves it towards the rear end of the gun which is open to the surrounding water. Water sitting behind the piston is pushed out of the rear end of the gun while a relative vacuum forms in the space opening up between the piston's front face and the rear face of the muzzle bulkhead. That means the spear is sitting in a vacuum inside the gun body when the gun is cocked and ready to shoot, the gun body being in a sense one big barrel. You can think of it as a scaled-up "Mamba" system, but without the rest of the gun.
The problem with this intriguing design is the large contact area of the piston seal and the need to have a very smooth finish on the interior of the gun body/big diameter barrel, otherwise sliding friction will absorb too much of the gun's power. Especially as abrasive particles like sand and grit can easily enter the gun and damage the finish on the internal bore of the big diameter barrel. Any loss of vacuum inside the gun and the gun will lack any shooting power, plus water will fill the gun body if it has a bad leak when the gun is cocked. After shooting the gun has no buoyancy, being full of water. It needs to have some mass or it will float when cocked, which is not such a good idea! The spear has no stop diameter on its tail and is attached to the gun with an auto-coil type shooting line affixed behind the spear tip.
Basically a quirky gun that explores yet another variation on the differential pressure theme which is used to power all pneumatic spearguns, regardless of the type.
This new version has a pistol grip which renders it more gun like in appearance and the color of the gun body suggests that it is made from titanium! A very big piece of expensive tubing!
One problem with this vacuum powered gun is that water can access the front end of the gun after the shot and if the piston is not fully forwards in the body tube then water will occupy the volume in the gap between the piston's front face and the rear of the front bulkhead thus shortening the operating stroke of the gun. If the front end of the gun progressively takes on more water with each shot the operating stroke of the piston will then shorten even further. One way to overcome this reduction may be already demonstrated in the video when the spear is shown being pushed in from the rear end of the gun as this would drive most of the water out of the front end of the gun by pushing the piston right up to and contacting the front bulkhead.
After the completion of the dive it would be necessary to wash both ends of the gun and then let it completely drain and dry out. Even so it does have the attraction of no spring or air pump, but the two seals in the muzzle and on the large moving piston respectively would need to be kept in good condition as would the surface finish on the bore of the body tube to maintain low sliding friction.
Unfortunately a bit of research has revealed that these guns are not very accurate as the big piston moving in the body tube/barrel tends to throw the gun around, including when it hits the muzzle at the end of travel. They were intended for spearfishing on scuba where the gun could be used at greater depths and make use of the higher ambient pressures, however they never lived up to their expectations and were no good against big fish. This may explain the big trident as that would increase the chance of a hit if the gun was not particularly accurate. The comments will be about earlier versions rather than this one with the big green grip handle, however it will be working the same way and the rapid movement of water rushing in through the back of the gun will also destabilize it. The only solution that I can think of would be to put two handles on it with a fore grip to control the gun better. If you think about it this is a bit like a slurp gun in reverse where you pull on the long metal rod connected to a big bore piston which sucks water into a long clear plastic tube and an aquarium sized specimen with it.