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Vacuum Muzzle sealing via the shock absorber body

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

Well-Known Member
Jul 30, 2008
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I am in the process of repairing my baby "Cyrano 55" as it popped its power regulator control shaft "O" ring after not having been used for about 5 years. I think the "O" has stuck on the 4 mm OD control shaft without being moved and being under full air pressure for all of that time, plus the gun does not seem to have much oil in it. Always held air and never leaked anywhere before until I recently moved the power regulator cursor to check it out, so I do not know where the rest of the oil went, but maybe it was never inside there in the first place. The oil certainly did not exit via the cursor control gate in the handle as it only leaked air when I pushed the cursor forwards and stopped as soon as I let it move back.

It occurs to me as I look at the disassembled muzzle assembly that there is another way to convert a "Cyrano" gun (and others) to pneumo-vacuum operation. Most conversion kits work on the muzzle nose end piece to create the inner barrel vacuum sealing system, but in the "Cyrano" and other guns with a removable muzzle nose end piece it should be possible to create the sealing system around the shock absorber body. What is required is an "O" ring in the thick rear plastic flange of the shock absorber body (it would require a groove machined there to take it) or one located at the back of the rubber sleeve against the step of the flange with the sleeve shortened to make space for the "O" ring which will seal the outer periphery of the shock absorber body in the smooth bore it slides into in the muzzle body and around the exterior of the shock absorber body itself. It needs to be a dynamic seal as the shock absorber body moves when it gets hit by the piston.

The shaft sealing "O" ring has to mount in the front end of the shock absorber body, which needs something to keep it there and maybe requires a new shock absorber body design at the front end. Now if you pick up a Mares big port muzzle and give it a shake you can hear a slight rattle as the shock absorber is not always tightly gripped, plus there is extra space between the front end of the delrin shock absorber body and the interior of the muzzle nose end piece which has a conical end well when looking inside it from the rear. This gap could be used to mount a rubber washer that controls an inner shaft sealing "O" ring located directly behind it, hence the new washer would not be a seal itself, but would restrict the "O" ring from moving off axis. There is more length in that well at the front than is required for the forward movement of the shock absorber under piston impact (but I don't know how much more), so some of the space could be used for fitting another component, especially a soft one. More rubber in there would add to the shock absorbing capability of the muzzle as right now any shock is taken by the flange at the rear end of the shock absorber body acting on the surrounding rubber sleeve. This system should work for shafts without a tail end stop diameter and if the "O" ring could be given some freedom to momentarily expand and let a shaft tail stop diameter through then it might work with them also. I see the benefit of this system is a shorter muzzle for pneumo-vacuum operation, in fact it will be the same length as for a wet barrel gun. Unscrew the muzzle nose and replace the shock absorber with the standard one and you could go back to wet barrel operation.

As Tromic has done the existing muzzle relief port holes would need to be covered with a rubber hose to seal them off. The screw in muzzle nose end piece would not need to be sealed on its screw threads as the necessary sealing would all be carried out on the shock absorber body located behind it. Something to think about anyway.
 
Well I did do my Cyrano with the Tomba it is a 70cm gun and it was an unqualified success, my only hitch is the fact that at close range the spear goes right through the fish...
 

Peter this is a sketch of what you are talking about. This is for 6,5 mm shaft. Just to have more realistic picture of your suggestion.



The placement of the groove for inner O-ring could be maybe lower, in the lower part of shock absorber.

Dimensions of the shock absorber body are not accurate (must be measured precisely). The length of the body is 25 mm, for Cyrano.
 
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This might be a body of shock absorber for 7 mm shaft for Cyrano. O-ring is 11 x 6 x 2,5 and 18.1 x 13.3 x 2.4.
It 26 mm long.

 
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I was actually thinking of the inner "O" ring being right at the front of the shock absorber, only partially sitting on a conical well rather like the taper that is already there. Right now that is just a lead-in taper on the front end of the shock absorber, although the shaft tail is directed by the bore hole in the muzzle nose-end piece before it gets to the shock absorber during shaft insertion in the muzzle. The plastic shock absorber body is probably thickest in section for a "Cyrano", but even so I would think that the two "O" rings, both outer and inner, should be separated lengthwise to keep as much material in the shock absorber body wall as possible. The rear outer "O" ring can sit in the space available if one cuts the width of the "O" ring from the rear of the rubber sleeve and inserts it against the step of the flange as I said earlier. Only question would be how it would stand up to being squeezed with each shot as it would then be a part of the shock absorber sleeve. Being pushed against rubber, i.e. the rear of the rubber sleeve, should not damage the outer "O" ring.

The front "O" ring could make use of the empty space currently between the shock absorber body and the muzzle nose-end piece to install something else there as well. The central shock absorber body moves forwards momentarily with each shot, so besides the "O" ring whatever else goes inside there has to have some fore-aft compliance, or enough to absorb any forward travel of the shock absorber body.

If you mount the gun's piston on the spear tail and slide the removed shock absorber body hard up against it then you will see that the stop diameter step on the shaft tail is nearly exactly lined up on the leading edge of the shock absorber. Now if an "O" ring located at this position in turn faced into a truncated cone tapering into the centre, the action of the shock absorber body coming forwards momentarily with the shot could be used to drive this "O" ring up the forward cone and spread it diametrically to let the shaft tail step through. This hard faced cone would need to be backed by a shaped rubber washer to give it concentricity and fore-aft compliance and also sandwich the "O" ring so that it had to stay on-axis with the barrel, these three new elements being in the gap between the muzzle nose-end piece and the shock absorber body. There would have to be space for the "O" ring to momentarily expand diametrically during the commencement of the passage of the shaft tail stop diameter through the "O" ring. This may sound good, I have not worked out the dimensions, but I expect that the inner shaft sealing "O" ring would have to posses a section or rubber width thick enough so that it would be engaged on the rearward facing front cone that would be there to spread the "O" ring. Alternately the step on the shaft could be used to stretch the "O" ring, but that requires the step not to develop any sharp leading edges over time that would damage the "O" ring.

Essentially the front "O" ring is to be controlled by the new nose elements rather than being bound by a locating groove in the shock absorber body itself, the tension on these elements being applied by screwing up the nose-end piece. A rod may be necessary to keep everything centralized as the nose-end piece is tightened. The intention is to retain as much of the existing muzzle as possible and only add new pieces and possibly a modified or new shock absorber body around about the same length as the standard one. If a new nose-end is machined then the shock absorber body can be lengthened, in which case both "O" rings could be installed in the shock absorber body as you show in your diagrams. It may be possible with the nose-end piece as is, particularly for shafts without a tail stop as there is no need for the front "O" ring to act as an expanding and contracting element which makes everything much simpler.
 
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Peter, maybe you could just cut-copy-paste from my drawing and construct your idea. It would be more clear.
 
I have drawn two quick sketches, the top one is for shafts with "no tail stop" and the bottom one is for "shafts with a tail stop". The foremost element in each set is a rubber washer with a profile that neatly fits into the taper ended well inside the muzzle nose-end piece and located directly behind it is the plastic positioning element for the front inner "O" ring. These plastic (probably made of delrin) positioning elements align on the bore inside the muzzle nose-end piece. On the top version the inner "O" ring is not meant to move, while on the bottom version it has the freedom to be expanded radially outwards. These are not scale drawings, so the "O" ring section widths are not the correct size, nor are the bore sizes through each part, but the diagram shows where the sealing "O" rings are to be located. The opposing cones on the bottom version are intended to squeeze the front inner "O" ring outwards when the piston first hits the shock absorber, that way the "O" ring's inner periphery will hopefully miss the potentially sharp leading edge of the shaft tail stop diameter as it passes through it. If the "O" ring falls back onto the cylindrical surface of the tail stop diameter there should be no problem, the idea is to just avoid hitting the leading edge. During loading of the gun the shaft tail stop step will be travelling the other way, so should not cause any damage to the front inner "O" ring. The rear step on the shaft tail is a tapered incline section, so it should not pose any problems for the "O" ring. I don't know if this "sharp edge avoidance" idea will work, but it exploits the forward movement of the shock absorber as the piston strikes the shock absorber's rear face and the spear tail immediately detaches sending the perpendicular tail stop step through the front inner "O" ring. This could not be achieved if the front inner "O" ring was part of the shock absorber, so that is why they are separated, but for the "no tail stop" version both "O" rings could be incorporated in the shock absorber body as it is a much simpler way to do it.

Hydraulic lock considerations may require some vents holes in the positioning elements to allow the front inner "O" ring to move, but the usual gaps around these parts may be sufficient to stop this occurring.
 

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I understand the basic idea of both concepts, not all details. It would be very helpful to have precise drawing in scale of all the pats in muzzle, including the shaft tail stop for second drawing. With existing pats of Cyrano I think the shaft stop diameter would hit the O-ring before the piston hits shock absorber.
 

Yes, it would hit the "O" ring because the step is now right at the front tip of the standard shock absorber, so that version's shock absorber body would need to be slightly longer. The length is OK for the shaft without the tail stop, in fact it could be shortened slightly to modify the existing front end to cup the "O" ring.

The rubber element in the nose could be a coil spring, it just needs to load up the front "O" ring to keep it centralized. The rubber element is shown in grey, I did not change all the lines, but the divider is shown by a grey line. The two "O" ring positioning elements could also be arranged like a mini-shock absorber, but I think the sizes would be too small.
 
This is a detail of drawing according to your second idea. Red line denotes where damage to O-ring might happen. With thinner O-ring situation is worse, with thicker might be better. I suppose it is not very good for O-ring to be very thick, than it stretches more difficult.



I gave Marko to make me one new type "TombaF7 easy" similar to my drawing: http://forums.deeperblue.com/pneuma...aling-via-shock-absorber-body.html#post859248

I am going to test it on the sea soon.
 
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Yes, the locations marked in red would need to be rounded off, it is exactly the same situation we discussed on the long running "Tomba" thread, in fact you had a drawing showing two cones facing each other with rounded off inner edges on one of the line slide variations. Now instead of being on a line slide the action is to be brought inside the confines of the muzzle nose-end piece. Radial stretch needs to be greater than 0.5 mm on top of the slight stretch already present in "O" ring as it embraces the body of the shaft. At least the jumping "O" ring cannot escape as it may have done in the line slide situation as there it was not surrounded by anything.
 
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One reason why this method of sealing may not have been used is that replacement of the shaft sealing "O" ring requires the gun to be depressurized. On vacuum sealing systems installed in the muzzle nose-end the front seal can be changed over with the gun still pressurized as the shock absorber is restrained by the extra muzzle component that is installed between the muzzle's main body and the nose-end piece, or it should be as otherwise that extra threaded muzzle component is not necessary. On a pneumatic gun with a power regulator it is possible to just depressurize the inner barrel and pre-chamber, this is a risky procedure, although I have done it unwittingly with the "Titan" and the Scubapro "Magnum" due to the reversal of the power regulator positions for "High" and "Low" power. It does not take long to pump up those spaces as the volume is much smaller than the main reservoir, particularly in a long gun with a big tank, but I would not recommend it as a servicing procedure as an unexpected pressurization of the inner barrel through a sudden leak in the power regulator system would send the piston flying out of the gun. The more frequently one had to change the shaft sealing "O" ring, then the less appealing this sealed shock absorber system would become, even though the muzzle is much shorter.
 
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The servicing problem could be overcome by splitting and essentially lengthening the screw threaded nose-end piece into two parts, an annular threaded bush that screws in to retain the shock absorber (abutting against the rubber shock absorbing sleeve) and a nose-end piece that screws in just in front of it using the same threaded bore. To maintain strength with the nose-end piece temporarily removed for changing the front inner "O" ring the muzzle body and its forward threaded internal bore would need to be lengthened by 7 mm, i.e. the length of the existing thread on the rear of the muzzle nose-end piece and which will also be the length of the new threaded annular bush. Hence installed back-to-back the threaded nose parts would engage the muzzle body bore threads over a length of 14 mm instead of the current 7 mm. Interestingly this length of thread (7 mm) is all that stops the muzzle nose-piece being blown out of the "Cyrano" gun followed by the shock absorber and the piston. The same probably applies for most other guns unless they use a one-piece muzzle where the shock absorber body inserts from the rear, then the task of holding everything together falls to the muzzle connection threads on the end of the inner barrel tube.
 
Another addition to the "sealed shock absorber" vacuum barrel system would be to insert a clear polycarbonate tube inside the muzzle between the front end of the inner barrel and the muzzle shock absorber anvil (or stopper). This clear tube would block off the muzzle relief ports internally thus offering four separate "view windows" for checking out whether water had penetrated the vacuum barrel to any degree after loading the shaft into the gun. The polycarbonate tube would be conical rubber washer sealed at the inner barrel end (using the entrance taper found on the alloy inner barrel tubing) and "O" ring sealed into the smooth internal muzzle section beyond the relief ports and just behind the shock absorber. The inner barrel attachment screw threads inside the alloy muzzle extend forwards about halfway across the relief port positions, so an "O" ring will not work at the rear end of the polycarbonate tube, but an "O" ring should work at the front end of the tube. Sourcing the required polycarbonate tubing size would be the largest obstacle to performing this modification, although a plastic rod could be machined to the desired size and length and then grooved for the necessary "O" ring. This system would make for a vacuum barrel gun using many of the existing muzzle parts and would provide a continual check on the muzzle sealing capabilities of the shaft sealing "O" ring, which is the one that will wear out.
 

One problem that I see now is that the tube would have very thin wall, about 1.5 mm. I made similar tube for another purpose on Cyrano and Sten 11.
 
tromic said:
One problem that I see now is that the tube would have very thin wall, about 1.5 mm. I made similar tube for another purpose on Cyrano and Sten 11.
Click to expand...

You are right, the OD is around 14.7 mm and the bore would be 11 mm for a "Cyrano" if it has to fit inside the standard muzzle under the screw threads. Basically the plastic tube bridges the gap where the relief ports are and effectively extends the inner barrel right up to the face of the shock absorber. A metal tube could be used, but then there would be no "sight glass" effect.
 

To check vacuum I use a thin wall (2 mm) rubber tubing over the muzzle ports. When the vacuum is OK there is a dent which is consistent all the time.
 
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