I have been asked to write this as a primer for those who know little about spearguns, and even less about "dry barrel" systems. This speargun arrangement should really be known as a "pneumo-vacuum" system as strictly speaking a "dry barrel" system is something else, the latter use an air-filled barrel. I will just use the term "vacuum barrel" in this article to keep it simple.
Pneumatic spearguns are usually cocked by hand pushing a spear from the muzzle end of the gun, thereby ramming a sliding piston in the narrow barrel tube back against the pressurised air in the reservoir or pressure chamber located immediately behind it. The pre-pressurised air in the gun provides the strong spring action that will throw the spear from the gun once this procedure is reversed. In some pneumatic spearguns the pressure chamber is either located physically behind the barrel or folded back around it in the form of an outer tank enclosing an inner barrel tube. Rear handle pneumatic spearguns are usually of the latter type and I will be referring to that type from now on.
When the trigger is pulled on these spearguns the piston is mechanically released and the pent up air pressure stored behind it drives both the piston and the spear shaft which is jammed into the front face of the piston down the barrel towards the muzzle end of the gun. This happens because the pressure acting behind the piston is very much greater than the pressure acting in front of it, which in a standard flooded barrel pneumatic speargun will be exactly the same pressure as the ambient pressure of the surrounding water outside the gun. The problem with all pneumatic spearguns is that the muzzle outlet on the gun has to be restricted in diameter compared to the diameter of the inner barrel tube for two reasons. The primary reason is the sliding piston needs to be held captive in the gun or it will escape along with the spear and all the compressed air will then rush out of the gun in a big cloud of bubbles! The second reason is that as the spear shaft is of a smaller diameter than the inner barrel's diameter then it needs to be reasonably tightly controlled at the muzzle end or it will flop around in the barrel at the muzzle entrance and not line up properly with the longitudinal axis of the gun barrel. So the diameter of the bore through the muzzle is usually not much larger than the spear shaft diameter, allowing only a slight clearance for any increased stop diameter or radial step on the extreme spear tail.
The consequences of the small bore in the muzzle and why it is a problem are now explained. The trigger mechanism released piston whizzing down the barrel is not only pushing the spear along in front of it, it is also driving whatever else has been sitting in the barrel in front of it. That happens to be water in the case of a fully flooded barrel speargun and water is completely incompressible. Not only is the muzzle bore restricted in diameter, it is nearly completely blocked off by the spear shaft running through it, so the water inside the barrel and surrounding the spear can only squeeze out around the small gap surrounding the shaft. This slowing of water escaping from the gun sets up a back pressure in the water column in the barrel and will start to oppose the air pressure driving the piston from the rear. If water could not exit the barrel at all through the muzzle opening then the force acting on the piston's face would instantly equal that acting behind it and the piston would then cease moving in the barrel until the hydrostatic pressure got in behind the spear tail and blew it free of the piston and out of the gun, followed by all the water as the piston got moving again. This situation would be the ultimate form of hydrobraking and needs to be avoided if we are to have a workable speargun. Hence speargun designers have made muzzle bores just large enough for water to exit around the shaft and incorporated additional muzzle relief ports to allow the water in the barrel to escape sufficiently quickly that a back pressure does not develop to any extent and thus hold the piston back. This is not a perfect solution and some of the energy stored during loading of the gun is expended through pushing the water out and causing it to change direction by exiting via holes directed to the sides of the gun rather than moving axially. What we are actually seeing here is the inner barrel tube working like a pump during the shot. It requires energy to operate any pump, energy that could have been used to propel the spear, but while we cannot get rid of the pump, we can do something about what it is pumping.
In order to eliminate the pumping workload in the barrel robbing energy from the shot, the ideal solution is to have nothing inside the inner barrel at all but the spear. As discussed earlier the inner barrel diameter is larger than the spear diameter, so there is going to be an annular volume inside the barrel with the gun cocked and ready to shoot. When the piston is pushed back in the barrel during muzzle loading of the spear, the water from the surrounding environment is sucked in through the muzzle bore and relief ports to occupy the space left in the barrel in front of the descending piston. If the muzzle bore can be dynamically sealed to the spear shaft and the relief ports completely blocked off or eliminated then the spear alone will be moved down the barrel and pushing the piston to the rear of the gun. Then a partial vacuum will be created in the inner barrel as the small amount of water already in there will only compress back from a vapour and droplet state to what it was before loading started when we eventually shoot the gun. Hence when the gun is discharged by pulling the trigger there will be no loss of energy due to the piston having to pump the inner barrel clear of anything else, so more of the stored energy in the gun can then be transferred to the spear. This raises the overall efficiency of the gun compared to standard pneumatic spearguns and that is why with an equivalent amount of loading energy they can do more with that energy in terms of propelling the spear from the gun. More energy translates into increased acceleration of the spear during the propulsion phase inside the gun and a higher resultant muzzle exit velocity after which it is only the shaft's momentum that carries it to the target. Alternatively one can back off the chamber pressure in the speargun and obtain a similar performance to before with less loading effort by exploiting the increased efficiency in that way.
Another benefit of the "vacuum barrel" system is that in the cocked state the piston's front face is only exposed to the external ambient pressure being transmitted to it through the body of the spear shaft from outside the sealed muzzle. The piston does not have that ambient pressure acting over its entire front face due to the vacuum existing in the inner barrel in a sense shielding it. As pressure is defined as force acting per unit area, the smaller cross section of the shaft compared to that of the inner barrel means that the force opposing the spear being ejected from the gun, which is imposed by the external environment, is greatly reduced and thus the gun will be less affected by operating at depths where the ambient pressure is greater. For example a 7 mm diameter shaft in a 13 mm inner barrel gives an area ratio of 49/169 (area is calculated by the radius r squared, multiplied by the value of pi p, but in a ratio we can cancel out the values of pi and the factor of 2 for the radius and just use the diameter values instead). This gives us a factor of 0.29, which is a significant reduction of the external force created by ambient pressure opposing the shot. For a 6 mm diameter shaft in a 11 mm diameter inner barrel the reduction factor is 36/121 which equals 0.30. The same shaft in a 13 mm diameter barrel gives a value of 36/169 which equals 0.21, so even better if you could only load a 6 mm shaft in a 13 mm diameter barrel gun without bending the shaft.
This "vacuum barrel" system sounds ideal, but has some drawbacks. If any of the muzzle seals fail then water is sucked into the inner barrel and you lose rather than gain shooting power as back pressure (hydrobraking) will occur in the barrel, the very thing that was to be eliminated. The "O" ring seals in the muzzle-to-line slide and line slide-to-shaft sealing system need to be in good condition and the spear shaft surface has to be free of rust, nicks and other flaws that could tear or scratch the "O'" ring that runs directly on the shaft. If this happens then you can forget about creating or even retaining any vacuum condition in the inner barrel!
The well known "Mamba" system from Maorisub is one way to create a "vacuum barrel" pneumatic speargun; regular viewers of this forum will have read of others. The sealed line slide, or muzzle blocking slide, that runs on the shaft and departs with the spear needs to be compact or its increased hydrodynamic drag will negate some of the advantages gained by slowing the shaft during its flight. If the muzzle sealing slide is too tight a fit in the muzzle then it takes energy for the shaft to knock it free, so that is another factor to consider if you fancy making one of these units yourself. Another option is to freeshaft or tie the shooting line to the front of the spear and eliminate the tail stop on the shaft, then all the sealing units can be left sitting in the muzzle while the spear heads off on its own.
Pneumatic spearguns are usually cocked by hand pushing a spear from the muzzle end of the gun, thereby ramming a sliding piston in the narrow barrel tube back against the pressurised air in the reservoir or pressure chamber located immediately behind it. The pre-pressurised air in the gun provides the strong spring action that will throw the spear from the gun once this procedure is reversed. In some pneumatic spearguns the pressure chamber is either located physically behind the barrel or folded back around it in the form of an outer tank enclosing an inner barrel tube. Rear handle pneumatic spearguns are usually of the latter type and I will be referring to that type from now on.
When the trigger is pulled on these spearguns the piston is mechanically released and the pent up air pressure stored behind it drives both the piston and the spear shaft which is jammed into the front face of the piston down the barrel towards the muzzle end of the gun. This happens because the pressure acting behind the piston is very much greater than the pressure acting in front of it, which in a standard flooded barrel pneumatic speargun will be exactly the same pressure as the ambient pressure of the surrounding water outside the gun. The problem with all pneumatic spearguns is that the muzzle outlet on the gun has to be restricted in diameter compared to the diameter of the inner barrel tube for two reasons. The primary reason is the sliding piston needs to be held captive in the gun or it will escape along with the spear and all the compressed air will then rush out of the gun in a big cloud of bubbles! The second reason is that as the spear shaft is of a smaller diameter than the inner barrel's diameter then it needs to be reasonably tightly controlled at the muzzle end or it will flop around in the barrel at the muzzle entrance and not line up properly with the longitudinal axis of the gun barrel. So the diameter of the bore through the muzzle is usually not much larger than the spear shaft diameter, allowing only a slight clearance for any increased stop diameter or radial step on the extreme spear tail.
The consequences of the small bore in the muzzle and why it is a problem are now explained. The trigger mechanism released piston whizzing down the barrel is not only pushing the spear along in front of it, it is also driving whatever else has been sitting in the barrel in front of it. That happens to be water in the case of a fully flooded barrel speargun and water is completely incompressible. Not only is the muzzle bore restricted in diameter, it is nearly completely blocked off by the spear shaft running through it, so the water inside the barrel and surrounding the spear can only squeeze out around the small gap surrounding the shaft. This slowing of water escaping from the gun sets up a back pressure in the water column in the barrel and will start to oppose the air pressure driving the piston from the rear. If water could not exit the barrel at all through the muzzle opening then the force acting on the piston's face would instantly equal that acting behind it and the piston would then cease moving in the barrel until the hydrostatic pressure got in behind the spear tail and blew it free of the piston and out of the gun, followed by all the water as the piston got moving again. This situation would be the ultimate form of hydrobraking and needs to be avoided if we are to have a workable speargun. Hence speargun designers have made muzzle bores just large enough for water to exit around the shaft and incorporated additional muzzle relief ports to allow the water in the barrel to escape sufficiently quickly that a back pressure does not develop to any extent and thus hold the piston back. This is not a perfect solution and some of the energy stored during loading of the gun is expended through pushing the water out and causing it to change direction by exiting via holes directed to the sides of the gun rather than moving axially. What we are actually seeing here is the inner barrel tube working like a pump during the shot. It requires energy to operate any pump, energy that could have been used to propel the spear, but while we cannot get rid of the pump, we can do something about what it is pumping.
In order to eliminate the pumping workload in the barrel robbing energy from the shot, the ideal solution is to have nothing inside the inner barrel at all but the spear. As discussed earlier the inner barrel diameter is larger than the spear diameter, so there is going to be an annular volume inside the barrel with the gun cocked and ready to shoot. When the piston is pushed back in the barrel during muzzle loading of the spear, the water from the surrounding environment is sucked in through the muzzle bore and relief ports to occupy the space left in the barrel in front of the descending piston. If the muzzle bore can be dynamically sealed to the spear shaft and the relief ports completely blocked off or eliminated then the spear alone will be moved down the barrel and pushing the piston to the rear of the gun. Then a partial vacuum will be created in the inner barrel as the small amount of water already in there will only compress back from a vapour and droplet state to what it was before loading started when we eventually shoot the gun. Hence when the gun is discharged by pulling the trigger there will be no loss of energy due to the piston having to pump the inner barrel clear of anything else, so more of the stored energy in the gun can then be transferred to the spear. This raises the overall efficiency of the gun compared to standard pneumatic spearguns and that is why with an equivalent amount of loading energy they can do more with that energy in terms of propelling the spear from the gun. More energy translates into increased acceleration of the spear during the propulsion phase inside the gun and a higher resultant muzzle exit velocity after which it is only the shaft's momentum that carries it to the target. Alternatively one can back off the chamber pressure in the speargun and obtain a similar performance to before with less loading effort by exploiting the increased efficiency in that way.
Another benefit of the "vacuum barrel" system is that in the cocked state the piston's front face is only exposed to the external ambient pressure being transmitted to it through the body of the spear shaft from outside the sealed muzzle. The piston does not have that ambient pressure acting over its entire front face due to the vacuum existing in the inner barrel in a sense shielding it. As pressure is defined as force acting per unit area, the smaller cross section of the shaft compared to that of the inner barrel means that the force opposing the spear being ejected from the gun, which is imposed by the external environment, is greatly reduced and thus the gun will be less affected by operating at depths where the ambient pressure is greater. For example a 7 mm diameter shaft in a 13 mm inner barrel gives an area ratio of 49/169 (area is calculated by the radius r squared, multiplied by the value of pi p, but in a ratio we can cancel out the values of pi and the factor of 2 for the radius and just use the diameter values instead). This gives us a factor of 0.29, which is a significant reduction of the external force created by ambient pressure opposing the shot. For a 6 mm diameter shaft in a 11 mm diameter inner barrel the reduction factor is 36/121 which equals 0.30. The same shaft in a 13 mm diameter barrel gives a value of 36/169 which equals 0.21, so even better if you could only load a 6 mm shaft in a 13 mm diameter barrel gun without bending the shaft.
This "vacuum barrel" system sounds ideal, but has some drawbacks. If any of the muzzle seals fail then water is sucked into the inner barrel and you lose rather than gain shooting power as back pressure (hydrobraking) will occur in the barrel, the very thing that was to be eliminated. The "O" ring seals in the muzzle-to-line slide and line slide-to-shaft sealing system need to be in good condition and the spear shaft surface has to be free of rust, nicks and other flaws that could tear or scratch the "O'" ring that runs directly on the shaft. If this happens then you can forget about creating or even retaining any vacuum condition in the inner barrel!
The well known "Mamba" system from Maorisub is one way to create a "vacuum barrel" pneumatic speargun; regular viewers of this forum will have read of others. The sealed line slide, or muzzle blocking slide, that runs on the shaft and departs with the spear needs to be compact or its increased hydrodynamic drag will negate some of the advantages gained by slowing the shaft during its flight. If the muzzle sealing slide is too tight a fit in the muzzle then it takes energy for the shaft to knock it free, so that is another factor to consider if you fancy making one of these units yourself. Another option is to freeshaft or tie the shooting line to the front of the spear and eliminate the tail stop on the shaft, then all the sealing units can be left sitting in the muzzle while the spear heads off on its own.