Evolution of a pneumatic speargun

Operating Principles

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 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 piston's 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 built 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 flow 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 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 piston's 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 inspection.

Evolution

Early pneumatic guns were mid-handled and evolved from not much more than a simple long tube, e.g. Nemrod "Torpedero", to the classic layout of a larger diameter rear air reservoir with small diameter barrel projecting forward of the centre handle. The Technisub "Jaguar" and Nemrod "Clipper" are typical examples, while the Russian "Seabear" (or "Pirometr" as it is locally known) is a more recent version. As such they were non-floaters because their metal construction far outweighed the volume of water they displaced. Later models increased the size of the air reservoir by extending it forward to cover the barrel, thereby improving loading characteristics as this reduced the gun's compression ratio. Plastics were increasingly being used as 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 fired. 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. All 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 free 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. 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. This resulted in a more efficient gun which also discarded the traditional line slide's spring shock absorber on the spear shaft tail for a rubber shock element in the line itself, coupled with a more hydrodynamic shape for the moulded plastic line slide. Its sibling, the third major revision of the "Sten", continues to use the standard size barrel allowing the use of heavier, larger diameter spear shafts, as do most of the other brands.

New Developments

Development continues however and floating 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. 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.

Hydropneumatic Spearguns

Another 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 "O" 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 air re-expansion forms a column which drives the spear shaft out of the barrel at high speed. The hydropneumatic gun is thus 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" can raise the 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.

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 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.

The following information for pneumatic guns.

Mares Mirage: 7 mm and 8 mm diameter spears, chamber pressure 40 ATM (max), barrel internal diameter 13 mm (uses a separate internal pumping barrel of 10 mm diameter to transfer air between internal chambers, otherwise 7 mm spears can bend in main barrel during loading at highest chamber pressure; a complex gun not manufactured for decades, many components).

Mares Sten: 8 mm diameter spear, chamber pressure 30 ATM (max), barrel internal diameter 13 mm.

Mares Cyrano: 7 mm diameter spear, chamber pressure 30 ATM (max), barrel internal diameter 11 mm.

All guns are rear handled and float on discharge, Mirage only made in 80 cm length model, all others from 40 cm up to 115 cm barrel lengths, however shorter and longer models have been made, the latter to special order. A special Sten was made with a titanium inner barrel of 155 cm length for "Pipin", a famous spearfisherman originally from Cuba. The big Sten shot a grouper at 30 feet range (about 10 metres).

Many other European brands are just variations on these Mares guns, cosmetic changes made only to vary appearance. Pneumatic gun efficiency is impaired by the piston pushing water as well as the spear from the barrel due to the muzzle opening constriction required to retain the piston in the barrel. The hydropneumatic gun does not need relief ports in the muzzle, but they are a necessity with pneumatic spearguns.

Peter Currer

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I have looked at your web-page with Dave Sutton's material. I have also viewed his web-site, very interesting material on many things, particularly old Russian diving gear such as rebreathers and underwater scooters. His "Airmatic" speargun is certainly in good condition. I have never seen one before, but I have seen the patent which described the gun very well. The inventor was John Salles, an American living in Orange, New Jersey. He applied for the patent in July 1955 but it was only granted in February 1960. The patent describes 3 versions, 2 of which are evolutions of the basic design, but it looks like he only built the basic version as the second and third versions were somewhat awkward, although the principles were sound. In these versions he investigated moving the release mechanism rearwards in the gun and overlapping the air reservoir tube with the firing tube (the barrel). This makes the gun more compact for a given spear length, a design principle used everywhere today, but with more modern trigger mechanisms. I do not know if Dave Sutton has explained how the old gun works, but here is a summary. The "Airmatic" has a simple layout to install a trigger which penetrates the air chamber without incurring sealing difficulties. A transverse rod with a semi-circular cut-out in it functions as both the sear lever and pivot pin. It penetrates near the top of the inner barrel to engage and block the piston's movement. When the trigger lever, externally connected to this rod, is revolved to the fire position the cut-out rotates and allows the piston's mushroom shaped tail to escape from under it. Many early pneumatic spearguns used this design of a side mounted trigger which had to be bent or curved laterally so that the trigger's tip lined up with the trigger guard on the speargun's centre-line. The rear air reservoir contains an in-built air pump to pressurise the gun before diving. The shiny handle of this pump screws into the blue rear section of the gun to seal it off from water penetration when the gun is in use. The pump handle is big and heavy to act as a counter-weight so the gun is balanced at the handgrip when the gun is loaded. The spear tail only penetrates as far as the handgrip, so the rear section is only the air reservoir and the in-built air pump. Thus the spear has powered travel for only the front half of the gun, (length of forward barrel) which means it is around one metre in effective length, still a fairly long pneumatic speargun. Projected later versions were to allow some of the barrel to occupy the rear air chamber, however the simple trigger mechanism used in this gun does not really lend itself to this arrangement. The piston has the shock absorber built into the piston, not the muzzle as many pneumatic spearguns do today. The original design was to hold a reduced diameter spigot on the extreme spear tail with a collet arrangement on the piston nose to retain the spear in the gun when loaded. This is similar to the type used on much later GSD spearguns which used a nylon piston to grip a small tail mounted ball on the rear of the spear. I do not know if the Airmatic used this in the production gun as I cannot see the spear tail very well in the photos. The other significant design aspect was the low position in the gun of the barrel with respect to the rear air reservoir. Note the centre-line of the barrel is closer to the grip than the centre-line of the rear reservoir section. This mismatched axial layout was to reduce recoil induced muzzle flip on the gun. The firing barrel axis is close in a vertical sense to the centre of grip pressure of the diver's hand, an important consideration ignored by many later designers. The rearward thrust on the gun is also low with respect to the gun's longitudinal axis, tending to revolve the muzzle down, or that is the theory anyway. The inventor said it practically nullified the tendency of the muzzle of the gun to rise during shooting. The axial displacement was about 0.5 inches. A very interesting speargun with many features of later pneumatic spearguns anticipated. The only things he did not include were muzzle relief ports to avoid water braking in the barrel, although the spear was a relatively close fit in the barrel, the muzzle being not greatly reduced in diameter compared to the barrel diameter as the shock absorber was in the piston. Modern speargun muzzles have to be large in diameter to incorporate the shock absorber, either a "top hat" with a rubber annular sleeve or a coil spring, though the latter is not often used today.

I forwarded my comments which I sent to you on the "Airmatic" speargun to Dave Sutton to see if there were any changes. He replied with the information shown below, so some aspects of "Airmatic" speargun were changed in the production models. Collet type line slide was abandoned for standard arrangement as was collet type piston to spear tail connection. Dave said spear tail had side-mounted, spring-loaded detent ball that clicked into annular groove located in the piston face. Some "Airmatic" owners also used Nemrod pistons with the usual spear tail taper and conic friction arrangement.

The piston has the shock absorber built into the piston, not the muzzle as many pneumatic spearguns do today. Actually, the production ones all have a large spring in the muzzle, no buffer was ever fitted to the piston in production guns. This was another of Salles inventions that never made it into production. The other significant design aspect was the low position in the gun of the barrel with respect to the rear air reservoir. Note the centre-line of the barrel is closer to the grip than the centre-line of the rear reservoir section. This mismatched axial layout was to reduce recoil induced muzzle flip on the gun. Which works superbly. A very interesting speargun with many features of later pneumatic spearguns anticipated. The only things he did not include were muzzle relief ports to avoid water braking in the barrel A feature which all of us add after the fact, by drilling holes in the muzzle to allow water venting on firing. Another thing we universally do is to remove the pump from the rear of the gun, and to simply use a high pressure cylinder and a hose to fill then gun. This effectively adds about 30% to the air chamber volume. We have also "folded" the gun, with air chamber parallel and over the barrel, to shorten them. These guns have bbeen the subject of much modification over the years, and soem good work came from it. Dave Sutton

Peter Currer ( September 2004 )

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