I note that there have been comments about the difficulty in pressuring pneumatic spearguns. While it might appear obvious to some it is worthwhile reviewing why the hand pump supplied with each speargun works. The pump only transfers air into the gun when the pressure developed inside the pump exceeds the air pressure inside the gun. With the gun fully depressurised air flows through the inlet valve in the back of the gun as the pressure created in the pump forces the inlet valve open, the valve being closed again by a not very strong spring, so not much effort required at all to inject some air into the gun initially. At first this pumping procedure takes very little effort, but once the gun starts to develop some significant pressure inside it then the pressure created in the hand pump has to overcome the valve spring and the pressure level already in the gun to open that valve. Also as the pressure in the gun builds the piston on the end of the plunger in the hand pump has to go progressively further down the hand pump's bore before the pressure developed in the pump body cracks open the inlet valve and the air transfers. You can think of a hand pump having a compression ratio dictated by its swept volume (the distance between the air breather holes near the top of the pump and the bottom of the pump stroke) and the dead air space at the base of the pump when it is fitted to the gun. If that dead air space is large then the gun can only reach a certain final pressure because no matter how hard or how many times that you pump it the pump can only reach the pressure dictated by the swept volume and the final space that all the air in the pump bore is being squeezed down into, i.e. the dead space. Consequently most hand pumps are manufactured to have a very small dead space to ensure that they reach a sufficiently high final pressure, however the gun manufacturer can limit the pressure in their guns by manipulating the dead space to prevent a higher pressure being achieved. To exceed this limit one would have to reduce the dead space volume by adding some solid material to reduce the available volume, however this is not recommended! Some older pneumatic guns had small reservoir capacities and when pumped up to high pressure were nearly impossible to load, so the manufacturer built a limit into the hand pump by leaving more dead space at the bottom of the pump.
Another way to pressurise pneumatic spearguns is to use a 12 volt air compressor sold for inflating vehicle tyres. These usually plug into the cigarette lighter socket in the dashboard of your car or they have some clips which attach directly to the car battery terminals. Some of these portable compressors have a pressure gauge fitted and will pump to 250 or 300 psi depending on the model. If you make an adaptor then you can use these compressors to pump your gun up very quickly to 20 Bar or 300 psi. as their pumping ability is only governed by the dead space in their actual compressor unit, not how they are actually connected to your gun. Those wanting much higher pressure in their gun can then switch over to the hand pump and add more air as more than half of the hard work has already been done by the electric compressor, although when starting up at higher gun pressures the hand pump needs a good push to drive it down and you will hear a squeaking noise as the inlet valve cracks open each time and the air transfers into the gun, usually close to the bottom of the hand pump stroke. This squeaking noise is a good indication that you are really getting somewhere, if you do not hear it then air must be going elsewhere and not into the gun. Do not worry, the gun will not blow up at these pressures, but it is worth checking that you can still load the gun, otherwise you will need to bleed some air out through the inlet valve. Once you have carried out this procedure a few times then you will start to get an appreciation of how hard the pump is to push as an indicator of the air pressure in the gun. Also when you finish any pump stroke that compressed air still in the dead space (some will be there as not all the air transfers once the pressure on each side of the inlet valve equalises) will push the pump handle back up. This pump handle movement is a rough guide to the pressure in the gun because when you let go of the pump handle the inlet valve has closed, in fact it closed as soon as you stopped pushing on the handle, and only the air pressure trapped in the dead space will re-expand to push the pump handle back up. Think of this functioning as a sliding column type pressure gauge because the non-transferred pressurised air still trapped in the bottom of the hand pump, in that dead space, has expanded to match the surrounding air pressure. That is what a pressure gauge does, except that there is some friction in the hand pump bore, but the handle movement is only a relative measure in any case.
Another way to pressurise pneumatic spearguns is to use a 12 volt air compressor sold for inflating vehicle tyres. These usually plug into the cigarette lighter socket in the dashboard of your car or they have some clips which attach directly to the car battery terminals. Some of these portable compressors have a pressure gauge fitted and will pump to 250 or 300 psi depending on the model. If you make an adaptor then you can use these compressors to pump your gun up very quickly to 20 Bar or 300 psi. as their pumping ability is only governed by the dead space in their actual compressor unit, not how they are actually connected to your gun. Those wanting much higher pressure in their gun can then switch over to the hand pump and add more air as more than half of the hard work has already been done by the electric compressor, although when starting up at higher gun pressures the hand pump needs a good push to drive it down and you will hear a squeaking noise as the inlet valve cracks open each time and the air transfers into the gun, usually close to the bottom of the hand pump stroke. This squeaking noise is a good indication that you are really getting somewhere, if you do not hear it then air must be going elsewhere and not into the gun. Do not worry, the gun will not blow up at these pressures, but it is worth checking that you can still load the gun, otherwise you will need to bleed some air out through the inlet valve. Once you have carried out this procedure a few times then you will start to get an appreciation of how hard the pump is to push as an indicator of the air pressure in the gun. Also when you finish any pump stroke that compressed air still in the dead space (some will be there as not all the air transfers once the pressure on each side of the inlet valve equalises) will push the pump handle back up. This pump handle movement is a rough guide to the pressure in the gun because when you let go of the pump handle the inlet valve has closed, in fact it closed as soon as you stopped pushing on the handle, and only the air pressure trapped in the dead space will re-expand to push the pump handle back up. Think of this functioning as a sliding column type pressure gauge because the non-transferred pressurised air still trapped in the bottom of the hand pump, in that dead space, has expanded to match the surrounding air pressure. That is what a pressure gauge does, except that there is some friction in the hand pump bore, but the handle movement is only a relative measure in any case.