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"Vacuum Barrels" aka "Dry Barrels"

Thread Status: Hello , There was no answer in this thread for more than 60 days.
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popgun pete

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Jul 30, 2008
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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.
 

foxfish

Silver Smoker
Dec 31, 2005
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Wow Pete, that is a very comprehensive & accurate description - I have made the thread a sticky so anyone can direct others who might need to know the absolute pros & cons of a dry barrel air gun.
Thanks.
 

Old Man Dave

Offline
Feb 19, 2005
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I agree, that's a great post. I thought that i would be clever and add a bit about free shafting and more especially about using the line tied to the front end of the spear so as to not need a line slider and rear end stop on the spear. However you even managed to onclude that as well. I think that having no end stop on the spear and trapping the sealing O rings in the muzzle is a big step forward. make everything so much simpler. Thing is does anyone know the full effect of having a line fixed to the front of the spear. Even on a band gun it would be much simpler to have the line fixed to the front instead of trying to fiddle with it inserting it into the trigger housing and getting in the way of the wishbones. Does a line fitted to the front spoil the spears flight? Be interesting to know.

Dave
 

foxfish

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Dec 31, 2005
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There seems to be a few methods of attaching the line, I have carried out a very simple option on my little mamba 90.
To use this method you will need to use a replacement threaded speartip & have a neat weld put in place about 150mm from the speartip.
I have not tried this method out yet & I have no idea what the effect of attaching the line so close to the spear tip will have with the range or accuracy?
I hope it works well as I really like the idea, one less component to fiddle with!
 

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popgun pete

Well-Known Member
Jul 30, 2008
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I have used one speargun with a front tied spear, it was not very accurate, although that was more to do with the bad workmanship in the machining of the muzzle's attachment thread to the gun (a Russian РПБ-1М). This topic probably deserves a thread of its own, however front tied spears are going to be steered by line drag as at the front it is more of a factor than if the line is attached at the rear end of the spear. The very earliest spring guns used front tied spears and I think we can deduce from the rapid adoption of line slides to these guns that a rear tied spear using a line slide system is more accurate. What can be done to improve the accuracy of a front tied spear is to use low drag shooting line and avoid the use of passive line clips to hold the line wraps in place.

Spearguns that have demonstrated a deterioration in accuracy with front tied lines during target shooting tests in swimming pools all used passive line holding arrangements where the shooting line has to pull itself off or out of the line release lever or clip. This jerk on the shooting line early in the shaft's flight probably caused some of the resultant inaccuracy. All the shooting line wrap loops being dumped before the spear tip moves away from the muzzle should eliminate this line jerking effect, however line running alongside a shaft has to have more steering effect than line trailing behind the shaft and travelling in its wake.
 

foxfish

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Dec 31, 2005
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Hmm food for thought - super thin braid & a stretchy bungee?
I am not going to sleep tonight!
I really want this method of line attachment to work for me :head :confused:
 

Cammo

Member
Aug 25, 2008
50
1
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Hi Guys,
That's a great wrap up Popgunpete on "wet" and "dry" pneumatics.
Just came across this ad, which is the first time I've seen a hydro-pneumatic for sale in oz. If its not you Popgunpete, you might want to add it to your collection. :martial
[ame="http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItem&item=250373511314&ssPageName=ADME:B:EF:AU:1123"]COBRA 500 Hydro-Pneumatic 50cm Spear Gun - New system - eBay, Spear Guns, Scuba, Snorkeling, Sport. (end time 24-Feb-09 14:13:09 AEDST)[/ame]

Some feedback from a recent "wet" pneumatic to a "dry" pneumatic user.
I've just gone from a standard Cyrano1100 and put on a Tovarich kit.
My initial opinion is that it's ended up a bit more complicated than it was before, but to be fair, this might be just due to the fact I now need longer shafts and have to use an "extended" loader, as I can't reach my hand up to the end of the spearshaft. Also I'm trying out a reel on the gun, so a few other complications.
Powerwise, going to the tovarich system it's hard to judge just by looking at the shaft flying out of the gun, really it would have been great to do some thorough tests side by side at the same pressure and have some concrete evidence. I know it should be more powerful, or (easier to load at a given pressure) so will stick with it. Also the concern about accuracy going down hill with the line attached at the front...yet to be determined.
A positive I have noticed is that the fish, once speared, now gets t-boned on the shaft instead of resting against the flopper when you pull the line, so it is more secure. Obviously this is due to the fact that the line is tied off around 150mm or so behind the flopper. This allows you to use a small flopper as there isn't a lot of concern about it bending against a fish when fighting it (assuming its not a kill shot, like all of mine, lol).
I also really see how it could be bad to use this system in caves as the spear doesn't come out end on, it comes out side on - lodging on everything! :friday
 
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foxfish

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Dec 31, 2005
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Hey Cammo, good on you for trying out a Tovarich & a front line attachment at that!
I have come across a few members who have not really felt the benefit of the dry barrel system, I must say that with my mamba 90s I immediately noticed a big difference in overall performance. Not only a power increase but a much smoother shot with less recoil, with my asso 115 the difference is not so noticeable in the power stakes but still a much smoother shot.
I really like my smaller size guns as they are still relatively easy to load but really pack a punch compared to a band gun of the same length.
 

popgun pete

Well-Known Member
Jul 30, 2008
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I have inspected a Cobra (Kobra) 500, it is a small forward latching speargun mainly used in freshwater (lakes and rivers) in its "home waters". As far as I could tell it has no stainless steel parts, everything is plated or bare steel if it is not aluminium or plastic (the handle, trigger and muzzle are plastic mouldings). Saltwater will make a meal of it unless you keep on top of washing it out thoroughly after a dive in the ocean, the new gun I examined even had a slightly rusty sear after (I assume) its factory check out test using freshwater! More a limited visibility speargun for poking around submerged obstacles and shooting whatever tries to escape. Quite a large number of them have been sold on eBay, I wonder how many have actually been used. Perhaps a subject for another post by someone who has tried one.

There is another version of the gun using many of the same parts, the pneumatic operation "Triton". The Kobra probably sells on having a quiet shot, being a collapsing rubber sleeve hydropneumatic like the RPS-3 (PПC-3) speargun, only a much simpler construction than the latter.
 

Cammo

Member
Aug 25, 2008
50
1
18
Ta for that PGP - my brother got all excited about it when he spotted it. I thought the price was too good to be true:blackeye
 

Old Man Dave

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Feb 19, 2005
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I missed that one. That is one great bit of engineering. Just goes to show that there is room for inovation. Dare I mention external top mounted barrel - again!

Dave
 

tromic

Well-Known Member
Aug 13, 2007
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Had anybody measured the real benefit of vacuum barrel concerning the speed or energy of the shaft? With my Cyrano 850 I could not notice significant difference: vacuum barrel-water barrel. But I had noticed a big difference with 6,5 mm havain shaft instead of 7 mm shaft equipped with the gun. The 6,5 mm shaft was longer than 7 mm shaft, and only 16 gr lighter.
 

Cammo

Member
Aug 25, 2008
50
1
18
Hey Tromic,
Just some anecdotal evidence - some mates were around on the weekend and were having a go loading my 1100 cyrano with tovarich kit. They couldn't believe how easy it was to load - so much so that they thought I had an air leak. We plugged in the pressure gage and it was fine. I guess might show that its a lot easier to load a tovarich kit on 25 bar than a wet barrel system? Really though I think loading effort should be the same.

A friend has a 8mm shaft in his pneumatic (can't remember what brand) and I got him to fire my gun (9/32" shaft in a cyrano 1100 with tovarich kit) and his at the same time. The 9/32" shaft seemed incredibly fast in comparison (which I would guess you would expect), really its hard to pick all this with just your eyes - we will video it next time.

I really wondered if there was any difference between wet and vacuum barrels, but now after using it for some time and having a go again on friends wet barrels guns I THINK it is superior. I would like to have some real evidence however!!
Cheers
Cam:blackeye
 

tromic

Well-Known Member
Aug 13, 2007
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Hey Tromic,
....
They couldn't believe how easy it was to load - so much so that they thought I had an air leak. We plugged in the pressure gage and it was fine. I guess might show that its a lot easier to load a tovarich kit on 25 bar than a wet barrel system? Really though I think loading effort should be the same.
....
I really wondered if there was any difference between wet and vacuum barrels, but now after using it for some time and having a go again on friends wet barrels guns I THINK it is superior. I would like to have some real evidence however!!
Cheers
Cam:blackeye
It could not be easier to load the gun with tovarich (vacuum barrel) then wet barrel. I have the opportunity of fast switching between vacuum barrel and wet barrel on my gun (see: tomba - all in one barrel sealing) and I am really not sure if there is a big difference in performance. It is somewhat more difficult to load the gun with vacuum barrel and the recoil is somewhat bigger, and the speed of shaft is somewhat bigger, but not easily noticable. I was trying to make some video and to compare but I should repeat taking the video with better camera.
 
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Cammo

Member
Aug 25, 2008
50
1
18
Hry Tromic,

How do they measure the joules of force that the spear comes out at?

This would show us the difference, or maybe penetration of a certain thickness at a specified distance?

It would be nice to know. The manufacturers of "dry barrel" kits all say you get up to 10 bar more preformance over a wet barrel system - which is a lot.

I'd like to put a standard tovarich kit against the tomba, I know it all looks good, but if the oring is not tight or too tight in any area performance will be way down.

On your video it looks to me like the 7mm shaft was better, but I guess that's just the camera angle.:)
 

tromic

Well-Known Member
Aug 13, 2007
1,672
206
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Hry Tromic,

How do they measure the joules of force that the spear comes out at?

This would show us the difference, or maybe penetration of a certain thickness at a specified distance?

It would be nice to know. The manufacturers of "dry barrel" kits all say you get up to 10 bar more preformance over a wet barrel system - which is a lot.

I'd like to put a standard tovarich kit against the tomba, I know it all looks good, but if the oring is not tight or too tight in any area performance will be way down.

On your video it looks to me like the 7mm shaft was better, but I guess that's just the camera angle.:)

Measuring penetration would be a good method.
I don't believe that dry barrel could get up to 10 bars. Suppose you make your barrel complete air free and that the ambient pressure is 1 bar. What should be the pressure inside barrel? -10 bars? I would say -1 bar.
I agree it is about camera and gun angle.
 

Cammo

Member
Aug 25, 2008
50
1
18
Hey Tromic (I watched Tron last night - 80's movie!)

When they sold me the tovarich they said a wet barrel gun at 30 bar is equivalent to a dry barrel gun at 20 bar - supposedly you are gaining an extra 10 bar in performance because you don't have to push the water out with the dry barrel system.
I would really like some measurements done, but need two guns exactly the same (and all my friends own 970's - not 1100's!). At least you can test with one gun! :friday
 

tromic

Well-Known Member
Aug 13, 2007
1,672
206
153
I was thinking about why there was not obvious difference in performance, at my video, vacuum barrel - wet barrel. I was loading the gun out of water. If I had loading the guns submerged in the sea I suppose the vacuum barrel (tomba) would be better then at he video. If there would be some water in the gun before loading, after sealing the barrel and loading the gun, there should be bigger vacuum in the gun because there would be less air in the muzzle at the begining. Some air could be also in the space between the piston and the muzzle. I suppose that mamba/tovarich could perform better because with the new muzzle without holes there is less air in the muzzle. So with tomba I should make less place for the air in the muzzle before loading the gun.
 
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