Revisiting the use of 7 mm shafts in pneumatic spearguns

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For many years the pneumatic gun had a 0.5 inch or 13 mm inner barrel and these guns fired 9 mm shafts with some using 8 mm in a few models. These were all Classic layout, rear tank guns with a mid handle. Then there was a transition period where the rear handle gun was introduced and these were fitted with 8 mm shafts and the guns became a lot lighter in weight because the great advantage was that they would float after the shot. A 9 mm spear was considered a bit heavy for these guns as with a heavier shaft you get more recoil if you increase the gun pressure sufficiently to power it. Some guns took on that challenge and used 14 mm inner barrels to throw those shafts, but they were not floaters.

The idea of shifting spearfishing attention from reef fish to open water swimmers and the introduction of band guns firing slimmer shafts caused a rethink on what could be done with a pneumatic gun. Some on seeing this shift in hunting foresaw the end of the pneumatic speargun. Muzzle loading puts a strain on the shaft, so Mares had the idea of loading a slim shaft in a smaller barrel while still shooting out of the standard 13 mm barrel. That gun was the Mirage and its main reason for being was that you could load and shoot slimmer 7 mm spears as an option to the 8 mm standard size. The guns were thus supplied with both shafts as standard equipment. However the multi-stroke loading system of the Mirage was slow with the transferring of shafts between barrels and the Mirage was a bit of a flop, as well as being expensive to make. Spearfishing was in the doldrums in the eighties with spearfishing being seen as "bad" when once it had been the bread and butter earner for dive equipment companies. Now their attention turned to scuba, dive travel and underwater photography while their spearfishing roots were hidden away.

Then in the early nineties, around 1994, Mares decided to create a gun that was dedicated to shooting 7 mm shafts and to use the same gun pressures, but put a smaller barrel in it at 11 mm diameter. Now these guns could be loaded without the risk of spear bending as had been the case with the earlier 13 mm inner barrel guns. That new gun was the Cyrano.

Now today this has all been forgotten as 7 mm shafts are being fired out of 13 mm barrels, but the force on loading can bend them. The answer is you have to lower the gun start pressure, but then you are still pushing water out and that is where the pneumo-vacuum gun comes in as with less force driving the gun you don't want to push extra water out that surrounds the spear, and as a consequence you can drop gun pressure. One factor that made slimmer spears a better proposition was making them out of better stuff such as spring stainless steel which was more robust than ordinary inox shafts and hence could be rammed in at relatively higher pressures.

To go any further on this trend would require a return to the Mirage concept which loads on a smaller inner barrel diameter and then shoots out of a larger one.

 

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The Mirage as introduced. The gun is fitted with its protective nose cap that covers the 13 mm muzzle. The 10 mm pumping and loading barrel has no muzzle as it does not require a shock absorber and sits just above the front line wrap hook. The rear line hook is a simple plastic clip which acts as a passive release. The original Mares shooting line was a thick solid core tow rope. The red plastic fold out loading handle in the butt has no upper tab at this stage in order to torment gun users trying to pull it out of the back of the grip handle while attempting bracing against their foot. More work was to be done on the gun, few liked it.

 

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The lower pumping barrel in the Mirage is shorter than the main barrel and has no shock absorber or piston catch, it is just there to move a shaft back and forth in with a non-return valve (inwards) at the rear and an expanding rubber sleeve valve (outwards) just slightly forward of that. The bore of the pumping barrel is 10 mm and that can take a tail stop at 9 mm in diameter which leaves next to no clearance, but as the shaft travels slowly when pumping it is unlikely to clip the walls and do any damage. By way of comparison a Russian Taimen has a 10 mm diameter shooting barrel which can swallow an 8 mm spear tail on a 7 mm shaft, but the key difference is a stainless steel inner barrel polished to the mirror, not just alloy as you have in the Mirage and Sten guns and the Cyrano models. As the Mares guns don't often get used in rivers and lakes they don't have to cope with gritty underwater conditions, whereas the Taimen does.

 

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Diving Gecko, whom we have not heard from for some time, built a new Mirage type gun using the Cyrano 1.3 handle to exploit its better breathing than the flawed innards of the Mirage handle which choked the airflow during the shot having tried to cram too much functionality into a short space.
You can read about that gun here;

https://forums.deeperblue.com/threads/geckosub-mirage-evo-and-adventures-in-3d-printing-speargun-parts.113094/

The problem with this "Mirage Evo'" gun is the carbon fibre tank, with two barrels in the gun it would be easier to stay with an alloy cylindrical tank, then one less thing to worry about when sealing the gun. Loading could be sped up by reducing the pumping barrel strokes to say two instead of five as by then most of the work has been done. The main barrel could be fitted with a vacuum barrel to shoot 7 mm spears out of its 13 mm barrel without putting too much bending on the shafts during loading.

Mares will not build a gun like this unless people ask for it, however they already have most of the parts, what they need is a new power regulator block for two barrels and a new two barrel exit muzzle, the top one being the shooting barrel.

As an 8 mm spear shooter the gun could be a direct counterpart to the Cyrano 1.3.

 

[popgun pete]

The 11 mm diameter inner barrel in the original Cyrano was designed to reduce the amount of water surrounding the 7 mm spear in the loaded gun. The mass of this water is not that significant compared to the weight of the spear, but it needs to escape from the inner barrel as the piston moves everything inside the barrel forward with the shot. Water is incompressible, thus water not outflowing as fast as the spear is progressing will slow the piston and the shaft down. Muzzle relief ports are used to provide an alternate pathway out of the gun as the gun's shock absorber has a diameter reduction in order to keep the piston from flying out of the gun. At one time relief ports were kept small to strengthen the muzzle and create a certain amount of hydrobraking to protect the muzzle from being destroyed by piston impacts as pistons were made of steel, as were the shock absorber anvils they struck.

When the first Cyrano appeared, pistons had already shifted to a plastic construction, so it was decided to increase the diameter of the muzzle relief ports as the muzzle strength could be marginally reduced due to lower impacts. Theoretically if the relief ports have a combined cross section that equals or exceeds the cross section of the annular column of water surrounding the shaft coming forward while being propelled by the piston this should ensure no hydrobraking in the barrel. However for the water to get out of the gun it still has to turn sideways through ninety degrees when it hits the shock absorber anvil face and that acceleration takes some energy out of the shot. One method to improve this situation is angled ports as the angular change in water momentum is not as great and that means less energy is required.

The ultimate removal of hydrobraking is the vacuum barrel design as next to no water surrounds the spear in the gun, but there is a slight added drag due to the friction of the vacuum cuff. With good design this drag is very low as long as the shaft tail stop diameter can clear the cuff without hanging up. There is a downside if the vacuum cuff leaks water as the inner barrel annular space will fill and create a large amount of hydrobraking. Omer used such hydrobraking in the Airbalete as a power control option and they claimed it took 15 percent of the gun's energy when the muzzle relief ports were rotated closed. It should be remembered that until the shaft stop diameter passes through the shock absorber body there is always a clearance around the shaft through the muzzle, but the clearance is of the order of 0.5 millimetre on either side.

 

[popgun pete]

Hydropneumatic spearguns don't have muzzle restrictions as there is no piston to fly out of the gun's inner barrel, therefore the spear tail stop diameter is a close fit in the inner barrel bore. But they do have hydrobraking deeper inside the gun where often an annular piston is used as a large water pump to blow the shaft out of the gun. That effects gun efficiency which is somewhat lower than a pneumatic speargun as the hydraulic system can be thought of as an intermediate element buried inside a pneumatic speargun. The guns are very quiet shooters as pistons don't move at high speeds in the gun, they can be thought of as a giant water pistol. Water speeds up inside the gun as it moves into a narrower cross-section represented by the barrel bore.

These guns usually fire slim shafts and use higher charge pressures in their air tanks to make up for the lower operating efficiency. Incremental loading is similar to using a hydraulic car jack, a spear can be pushed back and forth multiple times to have the same effect as a full length loading stroke, but only in the valve trigger operated guns.