vacuum barrel and dry firing

[dangermouse]

Hello
i was wondering if it's ok to dry fire a pneu with vac barrel if you lubricate with silicone oil around the barrel seal?
i read that in a normal pneumatic speargun the piston speed would be to fast without the water normally inside the barrel having to be pumped out and that it would result in mechanical breakdown, is this correct? I've heard about vac conversion kits for normal pneu, does such a kit contain a better cushion/dampener to avoid piston slam?
thank u in advance, and please don't bring up the safety aspect of firing from land, this is mostly for the science!

 

[Stamatis]

I have seen a video by Salvimar, where they dry-fired a gun. They just held the gun firmly with the spear tip touching the floor, and fired. The piston did not hit the end of the barrel, so no problem there. Generally I don't think it is a good idea. You reminded me that when I was a kid, I fired my dad's gun at a tree (lol) and it had no problem (I think!).

 

[popgun pete]

Dry firing a pneumatic speargun in air the spear faces virtually no resistance exiting the gun, whereas while a vacuum barrel gun has nothing in the barrel besides the spear that spear still has to push through the water when it exits the gun during an underwater shot. So it is not the same as dry firing the gun in air. The problem is cracks in plastic parts can develop without you knowing about them if you use the gun for shots in the air. Pneumatic guns have been designed to have a long service life provided they are used underwater where there is always some resistance to the piston pushing the spear.

 

[dangermouse]

If i where to try anyway, what type og gun is the most robust sulution? and do a vac gun have better cussion at the end than a normal pneu? because i rather not buy a vac barrel, its pointless on land and is more expensive.

 

[popgun pete]

I don't know of any robustness ranking of pneumatic spearguns for this purpose, internally they are all very similar and use the same construction materials and in their handbooks they all forbid shots being taken in the air.

 

[dangermouse]

there is allready an ongoing test with a cressi sl star 550 it has been pumped up to max and shot a bunch of times without problems, i will try to find the link.

 

[tromic]

Initial speed of the shaft would be about the same in water and in air, but I would not test it anyway in air at max pressure - especially not until the gun would eventually break. I did some speed measurements few years ago. Inicial speed of the shat in water was very close to calculated value in air.

 

[dangermouse]

interresting, thank you for the input

 

[dangermouse]

here is the test i was talking about: and:

 

[dangermouse]

again, "safety officers" please indulge me as this is mostly for science of how things work

 

[dangermouse]

i will soon open a tread on the safety aspect of this, please use this.

 

[tromic]

Initial speed of the shaft would be about the same in water and in air, but I would not test it anyway in air at max pressure - especially not until the gun would eventually break. I did some speed measurements few years ago. Inicial speed of the shat in water was very close to calculated value in air.

I find where I was measuring the speed of the shaft in water:
https://forums.deeperblue.com/threads/measuring-speed-of-the-shaft.91007/page-2#post-850903
Here is the image of results.

As you can see the initial speed of the shaft in water and in air is approximately equal.
The speed in water was measured after the shaft separated from the piston for Lo=20 cm.
The speed was calculated from travel distance dm=172 mm divided by time dt=5.16 ms.

 

[Mr. X]

Reminder: This forum is for diving and spearfishing.

 

[Crispin]

I dry fired my Cressi SL Star 70 at 30bar with a T0varich kit on it, it was a mistake, and it was a bad one. The Tovarich kit shattered into a gazillion pieces, and the piston was mangled as well.

Really - don't do it.

 

[tromic]

I dry fired my Cressi SL Star 70 at 30bar with a T0varich kit on it, it was a mistake, and it was a bad one. The Tovarich kit shattered into a gazillion pieces, and the piston was mangled as well.

Really - don't do it.

Crispin, if you were using that speargun with 7 mm shaft you might had the initial speed over 44 m/s.! Even with 8 mm shaft the speed might had been over 38 m/s! I would say that any initial speed over 35 m/s might be harmful for pneumatic spearguns - even in water.

 

[Philip32]

Where can I buy a dry barrel kit for my Seac gun?

 

[Crispin]

Crispin, if you were using that speargun with 7 mm shaft you might had the initial speed over 44 m/s.! Even with 8 mm shaft the speed might had been over 38 m/s! I would say that any initial speed over 35 m/s might be harmful for pneumatic spearguns - even in water.

There was no spear in it at the time, thankfully tromic, although it would have been 7mm yes, having used pneumatics solely since 1999 I'm extremely well versed in safety, and to this day I cannot understand how that gun would have been loaded out of the water or why I didn't test before I pulled the trigger - it bothers me....

The thing that upset me as well is that Ramon who made the kit has passed away now, he was a lovely guy to deal with so it was sentimental to me.

It's got an stc xpower kit on it now at around the same pressure, it fires fine in water, it's an absolute cannon in fact (secretly I love it more than my sporasub air one!), pistons are cheap enough - so i'll replace them if they fail I suppose.....I've had it since 2003 or so - it hasnt failed me yet so.....

 

[foxfish]

Is it possible to work out the ft lbs of energy produced from Tromics chart?
That is how fire arms are usually measured ... ie a .177 air riffle might be 12ftlb a .22 rimfire 80ftlbs 0r a high power hunting riffle might be 1000 ftlbs.
I would be interested to know how many ftlbs our pneumatics can produce...

 

[tromic]

Is it possible to work out the ft lbs of energy produced from Tromics chart?
That is how fire arms are usually measured ... ie a .177 air riffle might be 12ftlb a .22 rimfire 80ftlbs 0r a high power hunting riffle might be 1000 ftlbs.
I would be interested to know how many ftlbs our pneumatics can produce...

foxfish, it is in range (100 - 180) ftlbs for inicial energy and it drops to (15 - 20) ftlbs on (4-5) m.

 

[Diving Gecko]

Where can I buy a dry barrel kit for my Seac gun?

Most importantly be aware that your Seac has a slightly different thread pitch from Mares and most others (don't have my notes here but for 13mm barrels I think it's M18x1.25 vs M18x1). You can get the Evo-Air in that size. Tomba (Marko) can make you a beautiful one. STC should make one but I can't remember if the salvimar Vuoto is out in that size yet. LGSub might have one, too.


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