This article has been published in magazine "International Freediving and Spearfishing News" (Jan/Mar 2003 № 32) . www.diving-action.com
PNEUMATIC POWERED SPEARGUNS: ACCURACY AND POWER IN STREAMLINED PACKAGES
Pneumatic spearguns have been around since the 1950's, but many spearos still do not know much about them. This article aims to explain how they work and their evolution over the years which has made them the choice of many underwater hunters today.
The pneumatic speargun utilises a sliding piston which runs nearly the full length of a smooth surface barrel made from precision bore metal tubing, usually marine grade aluminium alloy, but stainless steel and titanium barrels have also been used on some models. The travelling piston is equipped with rubber seals to prevent any pressurised air escaping around it, especially when the piston moves along the barrel during the loading and firing operations. The spear tail usually jams into the pistons outer face using a conic friction arrangement, the slight taper on the extreme shaft end holding it in place until the gun is fired. On pulling the trigger the spear and piston move as a single unit along the barrel until the piston strikes the muzzle at the end of the gun, causing the spear to jerk free and travel on towards the target. The piston impact is countered by a shock absorber buill into the muzzle to prevent gun damage, but its effectiveness also relies on the higher density of water to slow things down and the braking effect of the water inside the barrel. Additional side ports built into the muzzle augment outward water ilow as the muzzle opening is always smaller than the barrel internal diameter. Firing a pneumatic gun out of the water will overload the muzzle due to excessive piston speed at impact and can cause severe damage to the gun's components. All spearguns should only be used when submerged and with great caution as it must be remembered that potentially lethal forces are at work.
The pneumatic gun is pressurised at the start of the hunting season with a special handpump to provide the force acting on the inner face of the piston which will be used to create the gun's "air spring". The higher this air pressure is, then the higher the "spring" force will be, however the loading task becomes progressively more difficult if the pressure is set too high. This can result In inability to load the gun in the water and may result in bending the spear if the pushing effort by the diver moves too far off the loading thrust line into the barrel as the diver struggles with it. The guns are generally pumped up to around 20 ATM, but more powerful models have the capability to go to 40 ATM and sometimes higher. Loading involves inserting the rear of the spear shaft into the gun's muzzle and pressing down on the spear tip using a special loading bar to protect the diver's hand. This causes the piston to be forced down the barrel against the opposing air pressure in the gun. Considerable effort is required to do this, particularly as the air pressure in the gun increases the further the piston moves down into the barrel. Air displaced from the barrel is progressively moved back into the main air reservoir, which consists of a tank either at the rear of the gun or surrounding the barrel and running forward to the muzzle. When the spear is nearly pushed right in the pistons inner end engages a sear lever which snaps over a mushroom-like projection on the back of the piston, holding it against the force of the compressed air trying to push it back out again. This is equivalent to the notch on a band gun spear shaft tail, but as the piston can revolve in the barrel during loading it needs to be able to be engaged by the sear at any orientation. The trigger operates through an air-tight seal to push (or pull) a small diameter rod that works against the sear lever arm to tip it to a point where the piston is released from its grip, thus firing the spear from the gun. No air is lost from the gun during this operation, and guns can retain pressure for years with little loss, but they should be depressurised for long-term storage, maintenance and internal inspection.
Early pneumatic guns were mid-handled and evolved from not much more than a simple long tube, e.g. the Nemrod "Torpedero", to the classic layout of a larger diameter rear air reservoir with small diameter barrel projecting forward of the centre handle. Guns usually came in a range of lengths by the simple expedient of lengthening the air reservoir and barrel tubes which allowed correspondingly longer spears to be used. The longer the gun the further the range was for effective shooting, plus greater impact was achieved at the target due to increased momentum with the longer and thus heavier spear shaft. The Technisub "Jaguar" and Nemrod "Clipper" are typical examples of these guns, while the Russian "Seabear" (or "Pirometer" as it is locally known in Russia) is a more recent version. As such they were non-floaters because the metal elements in their construction far outweighed the volume of water they displaced. Later models gradually increased the size of the air reservoir by extending it forward to ultimately cover the entire barrel, thereby improving loading characteristics as this reduced the gun's compression ratio. Plastics were increasingly being used tor structural elements in the air reservoir end sections, which meant that the main metal components could be reduced to the muzzle, trigger mechanism, piston, inner barrel tube and the air reservoir tube which formed the main outer body of the gun. The original Mares "Sten", a rear handled gun, adopted all these changes and revolutionised pneumatic guns by being light enough to float once the spear was tired. Once it arrived the heavy classic guns gradually fell from favour and the derivatives under numerous brand names virtually dominate the pneumatic gun market today. European brands such as Cressi-Sub, Omer, Beuchat, Technisub, Tiguillo and Seac Sub have offered their own versions of this design with detail improvements over the years, but the basic layout has stayed the same.
VARIABLE FIRING POWER
All the "Sten" type guns have dual power versions which allow part of the main air reservoir to be closed off by an external lever or "cursor" which operates a valve in a partitioning inner bulkhead just forward of the gun's rear handgrip. A separate non-return valve allows displaced air to flow into the forward section past the bulkhead during loading of the spear, but not out if the cursor operated valve is closed, so the gun fires the spear at a lower power because the only air that can expand is that in the rear section of the gun. Full power shots are accomplished with the cursor valve open as then all the air in the reservoir is tree to expand and drive the piston down the barrel, not just a reduced part of it. This feature also provides an "easy loading" capability because closing the cursor valve means that some of the air displaced during unsuccessful loading attempts is trapped behind the bulkhead, thus allowing the spear to be inserted using a number of separate strokes before the sear is finally able to latch onto the piston. This process spreads the loading effort out, the initial effort to move the piston is less each time, but the final force application with the loading bar required to achieve sear latching is unchanged as the air pressure in the gun will be uniform throughout regardless of the partitioning system. Opening the cursor valve then allows the front and rear air reservoir sections to communicate again if a full power shot is desired, but if a low power shot is taken with the cursor valve shut then the diver only needs to recompress the air in the rear section again, a somewhat easier reloading task, although the final effort just before the sear hooks onto the piston will be unchanged as before.
Modifications to the basic rear handle design over the years have improved corrosion resistance and optimised components for lighter weight and reduced manufacturing cost, while improving reliability, but older models are not a great deal different in operating characteristics to the later versions that succeeded them. Metal pistons have given way to plastic versions (they still require a metal tail to engage the sear catch) which have reduced barrel corrosion from electrolysis and allowed lighter components to be used in the muzzle itself. One significant change however was the adoption of an 11 mm internal diameter barrel in the Mares "Cyrano" to allow lighter "speed" spears to be used while minimising the water volume in the barrel compared to that if a standard 13 mm barrel was used. The gun also used much larger muzzle relief ports to assist outward water flow. These changes resulted in a more efficient gun which also discarded the traditional line slides spring shock absorber on the spear shaft tall for a rubber shock element in the line Itself, coupled with an improved hydrodynamic shape for the moulded plastic line slide. Its sibling, the third major revision of the original "Sten", continues to use the standard size barrel allowing the use of heavier, larger diameter spear shafts, as do most of the other brands offering this style of gun.
Development continues and floating (after spear discharge) mid-handle guns are starting to appear which are somewhat similar to the "Sten" concept, but with part of the air reservoir now extending behind the handle again, Strictly speaking they are more rear biased rather than mid-handled, similar to the situation with so called mid-handle band guns. The increased size of the air reservoir further lowers the gun's compression ratio, Improving their loading characteristics when trying to latch the piston at the bottom of the spear insertion stroke. Not as simple in construction because the outer body tube is In two sections of necessity to accommodate the '"centre" handle section, they use lightweight materials to achieve flotation, unlike the Mares Titan" of years ago which had a similar appearance but was a definite non-floater. The Russian "Neptun" and "Miron" are two examples of these latest designs and are available in varying gun lengths. The "Neptun" also comes in a full titanium version, making it a rather exotic weapon tor the 21st Century's underwater hunter. Even the fish should be impressed!
Another pneumatic powered variant is the hydropneumatic gun, which dispenses with the travelling barrel mounted piston found in the pneumatic gun. Instead the spear tail Is provided with an "0" ring which seals against the Inner barrel surface, taking over the function of the standard pneumatic gun's piston. On diving the barrel of the gun is allowed to flood with water, and inserting the spear during loading forces this water from the barrel into the body of the gun. This effectively reduces the internal volume of the gun and thereby raises the air pressure in a similar fashion to the pneumatic gun's operation. An internal valve at the back of the barrel lets the water In until the loading effort on the spear stops, then it closes, trapping the water inside the gun. The flooded parts of the gun are separated from the internal compressed air reservoir by a moving bulkhead in the form of an annular piston or a flexible membrane behind a ported grating, the latter being used to restrict the membrane's movement when the gun is in a discharged condition. Pulling the trigger opens the internal water valve, the expanding compressed air then pushing the internal moving bulkhead back to its original position from which it had been displaced by the loading operation. As water is incompressible, the volume displaced by the compressed air re-expansion forms a column which drives the spear shaft out of the barrel at high speed.
The hydropneumatic gun is essentially a pneumatic gun with water forming an intermediary transmission element between the spear shaft and the variable volume compressed air reservoir. It has the advantage that the injection of additional water through what is known as "hydropumping" with the spear or a separate water injection pump built into the gun can raise the gun's internal air pressure beyond that achieved by a single spear loading stroke. Hydropneumatic spearguns are not well known today, but have been around for as many years as pneumatic designs. Modern versions are available from countries such as the Ukraine and Russia, with the "Aquatech" being one of the latest designs.
ADVANTAGES AND DISADVANTAGES
All pneumatic powered spearguns provide axial drive to their spears for the full length of the gun barrel and are also very efficient converters of loading effort to spear propulsion. They are also very accurate as they are truly closed track guns, however they are limited by the diver's ability to load the gun if very high air pressures are used in the internal reservoir. Unlike multi-band guns the loading effort cannot be split by separate loading of each band, but they can be loaded in steps which give the diver time to pause for rest between successive pushes on the loading bar. Long limbs and considerable strength are needed if the longest guns are contemplated as the diver has to have the necessary reach to span the distance between the spear tip and the gun handle. An exception is the hydraulic triggered "Aquatech" hydropneumatic gun which allows the spear to be inserted to any depth in the barrel before any real loading effort is applied, as water can be diverted from the gun's interior through operating the trigger valve which releases it to the environment. Hydropumping using successive partial spear strokes can load these guns up to the diver's strength limit, a unique feature in pneumatic powered spearguns.
Air leaks from all pneumatic guns are avoidable if they are washed in fresh water after each use and sand is kept out of them. When in the water shaking them with the barrel pointed downwards before loading the spear will keep any highly abrasive stuff out, and if they are dropped to a sandy bottom while loaded it is a good idea to also do this before firing the gun. Any particles then left in the muzzle will soon be blown out by the water exiting the barrel as the piston and spear are moved rapidly to the front of the gun. Buying a pneumatic speargun will save on buying new rubbers each season for a band gun, but make sure you take your handpump with you on a trip, only try to avoid pumping your gun up on the beach as sand often can get in where you do not want it!
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