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Bluewater Pneumatics - Pros and Limitations

popgun pete

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Jul 30, 2008
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The safety aspect of the "Mirage" system is that loading using what is a gas transfer pump removes pressure from the inner barrel and the pre-chamber connected to the rear of it. The transferred gas is moved into the front tank where it is isolated by a manually operated valve. That means spear insertion into the charged gun faces no appreciable pressure, in fact it can be pushed in with next to no effort.
 

quakeos

Member
May 24, 2017
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Hi guys, I've been very busy lately but rest assured I haven't had a day where I don't mess with a pneumo. DiverGazzand I did some testing recently with a bunch of airguns with some really satisfying testing results. By the way- the new cressi saetta pro is a solid new model. I believe they are using a version of the STC-X muzzle and it appears to be a substantial upgrade from the salvimar vacuum muzzle. By the way - the muzzle on the saetta is cross-compatible with the predator vuoto.

Anyway, I've been giving a lot of thought lately into making the ideal pneumatic piston. I have concerns with using many metal materials, as this could cause galvanic reactions between the piston and the aluminium inner barrel - especially as we have water acting as an electrolyte between the two.

Any thoughts on this?
 

quakeos

Member
May 24, 2017
25
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I do also think there could be a breath of hope as the o rings on the piston will effectively isolate it from the inner barrel
 

Diving Gecko

shooter & shooter
Jun 24, 2008
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Titanium is the remedy for your galvanic corrosion issue.

I did see Gazz rave about the Saetta elsewhere but there were no details. And honestly I can’t picture it either based on your description especially anything STC being Vuoto compatible is mind boggling without pics.

Can one of you please find the already existing Saetta thread and update it. I’m really interested but would be cool to have the info in the most relevant thread.
(Found it: https://forums.deeperblue.com/threads/cressi-sub-saetta-pneumatic-speargun.113774/#post-981096)

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Diving Gecko

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Jun 24, 2008
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I do also think there could be a breath of hope as the o rings on the piston will effectively isolate it from the inner barrel
Nah, don’t count on that. Plenty of examples of just a tiny bit of water making it past the piston seals without the user noticing only to completely wreck meta parts in the gun. Ok if you service and check your own guns often. Not ok if you don’t.


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quakeos

Member
May 24, 2017
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Mm, titanium is the best solution I came up with aswell, it appears to have a reasonable compatibility with aluminium . I also pondered pultruded carbon fibre
 

Diving Gecko

shooter & shooter
Jun 24, 2008
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Pultruded won’t cut it. It’s basically 100% unidirectional and I suspect it would bulge and spilt on impact. Good for tension but don’t think it would work that well in compression.

Oh, I was thinking of tubes. As a rod, perhaps. Gat-ku uses it for their front elements. But also, pultruded is the cheapest way to make rods and tubes so there’s a lot of shitty ones out there. But as far as I recall you’ll get worse corrosion issues if using carbon. Graphite and alu don’t play well together, I think. If separated with a layer of glass or epoxy, it’s much better.

Also of importance is to keep the piston light and the friction fit holding the shaft to an absolute minimum. If not, you have higher impact forces than you need to have.




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Diving Gecko

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Jun 24, 2008
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Only issue with titanium is that’s it’s not that easy to cut on a hobby lathe. Ideally, you’d have flood cooling as it runs super hot. But I’ve managed to make a few parts and would still make shock absorbers and pistons in titanium if needed. Just not easy nor fun to cut.


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Diving Gecko

shooter & shooter
Jun 24, 2008
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By the way - the muzzle on the saetta is cross-compatible with the predator vuoto.
Ah, got it now. Yeah, well there are pretty much only two standards in threads for muzzles on 18mm barrels and Cressi has always(?) used the old "Mares standard" of M18x1.25 and so does Salvimar hence why they are all compatible/interchangeable. The one to really look out for is Seac as they use M18x1.5.

By the way- the new cressi saetta pro is a solid new model. I believe they are using a version of the STC-X muzzle[...]
I found a nice review on an Italian site and they do indeed say that the Saetta uses STC vacuum seals. Can't say how much the rest of the muzzle and shock absorber is like an STC without seeing the Saetta guts, but from the outside it looks a whole lot shorter and simpler. And they have slimmed down the tang enough to, hopefully, make it work with the STC seals which I suspect were designed only/mostly with front-tied shafts in mind.

I still don't like the super old school line release and the raised shooting barrel is not only a positive. Well, for tiny targets it does help with aiming but for a high powered gun it effectively lowers the handle, getting your hand less in line with the recoil forces...

Also, keep in mind that Cressi say not to go over 20 bar for their guns. A bit strange, actually but that's in their own documentation.
 
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popgun pete

Well-Known Member
Jul 30, 2008
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Pneumatic guns used metal pistons for decades, zinc then cadmium plated steel and later stainless steel. My Scubapro Magnum has a stainless steel piston and my Cressi SL has a steel piston. You can forget about inner barrel corrosion as it takes years to have any effect and that is just in the section of the barrel where the head of the piston sits up to the front seal because that area is the last to dry out. Theory is one thing, but reality is another.
 

quakeos

Member
May 24, 2017
25
7
18
18
Sydney
Pultruded won’t cut it. It’s basically 100% unidirectional and I suspect it would bulge and spilt on impact. Good for tension but don’t think it would work that well in compression.

Oh, I was thinking of tubes. As a rod, perhaps. Gat-ku uses it for their front elements. But also, pultruded is the cheapest way to make rods and tubes so there’s a lot of shitty ones out there. But as far as I recall you’ll get worse corrosion issues if using carbon. Graphite and alu don’t play well together, I think. If separated with a layer of glass or epoxy, it’s much better.

Also of importance is to keep the piston light and the friction fit holding the shaft to an absolute minimum. If not, you have higher impact forces than you need to have.




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Yeah , it was quickly eliminated for me as looking into it more not only is carbon and aluminium not very friendly but some articles and studies floating on the web suggests that pultruded carbon fibre has more poor compressive strength than standard twill carbon fibre. Fun fact - the rob allen samburu uses pultruded carbon fibre tubes.

And absolutely with piston lightness and impact force - it’s going to be short as practically possible and I am also considering making a centre piece insert of delrin or UHMWPE for less friction for further less impact force for that brief moment where the piston hits the shock absorber with the piston still grabbing the tang.
 

popgun pete

Well-Known Member
Jul 30, 2008
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The Scubapro Magnum was built using the best materials at the time and has a very good piston. It was created as an upmarket version of the Sten with thicker and stronger parts and contains no brass items, whereas many guns still contain brass today (e.g. rear inlet valve body). With a high purchase price it was superior to the later Vintair. The shock absorber is stainless steel in a one piece muzzle. The piston is short, has three running surfaces to stop it wobbling in the inner barrel and is not overly heavy. It was costly to make as it was only made for that gun and went against the trend to plastic at the time.
Scubapro Magnum piston.JPG

Magnum exploded parts.jpg
 

Diving Gecko

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Jun 24, 2008
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Those old steel pisons, did they ever have plastic inserts to hold the tang?
I think I only have one that came with an old Mirage, but don't recall it having an insert. Quakeos mentioned it and I have been pondering it before but couldn't come up with a good way to keep it in place. Best idea was to thread it in place but still not convinced it would work that well.
 

popgun pete

Well-Known Member
Jul 30, 2008
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The earliest pistons were fabrications of steel and aluminium with a steel spine, but they become all steel in the Mares guns of the sixties. The tapered spear tail jams in the hole in the front of the piston which is just a cylindrical bore in the steel head. Those pistons were cadmium plated which gives a dull golden colour with a slight rainbow effect in places and slowly corrodes away. Cadmium plating was widely used for many applications, but the process produces noxious solutions as cadmium is a heavy metal and a poison which used to be dumped in the ocean via drains. Cadmium was replaced with a dull silver finish which like cadmium is slowly eaten away, it did not chip or flake off as it does with some plating like flash chrome plating. The pistons were all in one piece with grooves for the "O" ring and a cone type rubber seal front and rear which were pushed on and stretched to sit snugly in their seats. The front cone seals were eliminated by replacing them with a plastic ring that was cracked so that it could be sprung into position, this ring being a short annular band. You can see such a front band on the Scubapro Magnum piston. The Mares pistons used in the Sten and Titan and Vicojet guns used the same arrangement before the move to plastic pistons with steel tails. The plastic annular rings/bands used to be grey in colour and they were used in the Mirage on both the main piston and the pumping barrel piston. The three contact areas, the rear rubber cone seal, the mid "O" ring and the plastic band stopped the piston wobbling in the inner barrel.

The Mirage piston shown here displays the three rubber seal piston, note the small hole to prevent hydaulic lock when the shaft tail jams in the piston.
Mirage CRX.jpg
metal piston.jpg

As the plastic pistons tend to stretch slightly, unlike metal, the small hole for hydraulic lock prevention disappeared.
 
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Diving Gecko

shooter & shooter
Jun 24, 2008
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@quakeos On another thread, a user just posted this (I put the main thing in bold):

Mate that first sentence in my last post was a quote from you. That basically refers to an illegal type of speargun which is loaded & cocked mechanically & then the spear is inserted.
Do this in Australia & you face a mandatory sentence of five years.
Mandatory means that is what the judge must sentence you too as minimum.
Imo you have already made a mistake by posting that sentence & you should understand why people are cautious before making further comments or proceeding with “developments”.
I don't know if this is correct but he sounds cetain. For experimental use, I don't know if that limitation is still in effect - but you may want to look into this. Especially if you had in mind to offer guns or modification sets for sale down the road.
 

popgun pete

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Jul 30, 2008
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Australia
I have not read the regulations in recent years, but I think the distinction is where the source of loading energy comes from. For example say you had a battery powered winch or pump to load guns, well that would not be allowed as you may just press a button and incur no effort. Air rifles are a grey area and some are cocked by a lever system when you break the gun on a central hinge and thus they are manually powered and in many places, including here, are controlled weapons. For a number of years pneumatic guns were not defined separately and at a loose interpretation were included in the definition of air rifles. However pneumatic spearguns had been allowed in because the old-timers in Customs knew what they were. When they retired a new bunch of officials decided to interpret the rules by the letter and put the brakes on any pneumatic gun. This caused many importers to tear their hair out as some self-styled judges had all their spearguns seized at Customs Inwards. Eventually after many to and fro letters it was decided, albeit reluctantly, that pneumatic spearguns were not air rifles. It was a close run thing, but aided by the precedent with all the Nemrod and Mares guns that had been here from the late fifties, although not in great numbers because spearfishing was a fringe sport pursued by lunatics who would soon be eaten. What changed was spearfishing was growing at the same time as firearm restrictions were being imposed. In the sixties you could buy .22 semi-auto rifles and pump action shotguns with no questions and in some cases no licence. That all changed as guns became “bad” after an increased use by criminals and psychopathic mass shooters and as those weapons were swept away a beady eye went onto spearguns, but for now the heat is off again.

The danger is if a super powerful gun was made that could be loaded by anyone and was fired in air during some altercation which ventilated a luckless victim then spearguns would become "bad" and the axe would come down on the sport. Right now spearfishing with its low profile does not attract dragon-slayers looking to make a name for themselves by shutting down spearfishing.
 
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popgun pete

Well-Known Member
Jul 30, 2008
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Australia
An interesting side note is that in an attempt to decouple spearfishing gear from regulations which related to firearms and the terrestrial weapons that drive most of the prohibitions imposed by governments there was the hydraulic powerhead produced by Sea Hornet. The standard 12 gauge powerhead for bumping off sharks was basically a cylinder with three sections, the rear body, the mid-bulkhead and the removable front barrel that screwed off. In the hydraulic version an extra section was added that had ports incorporated and this allowed a flooded section that transmitted the push imposed by the spear or pole that the powerhead was connected to, but only when the section was completely flooded. On land it would not work as the space inside just compressed trapped air and nothing happened. Unfortunately the general ban on explosive powered underwater weapons was sufficient to get rid of them as well despite this innovation. However spearguns that only work underwater was probably why hydropneumatic guns were allowable in many authoritarian countries that normally discourage weapons in the hands of civilians. An example would be the RPS-3 as with no water inside it the gun will not shoot.

It was this line of reasoning that had the "Black Sea" gun pried free of the clutches of Customs who had hung onto it for two months as without water the gun cannot work.
 
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