Well here is a bit more info. In fact a lot more info! Once I got into the reservoir on the newer RPS-3 gun I could see how it all went together, whereas the front end bulkhead was always too tight to remove on the partially dismantled and damaged RPS-3 gun and I could never figure out how to undo it without completely trashing it, so all I could do was pull the inner barrel out and look inside the reservoir tank from the rear end. Fortunately that stubborn front bulkhead also yielded to the special tube spanner I made as the twisting effort required was transmitted at four locating points, not one as it is with the standard "C" spanner that was supplied with the RPS-3 gun. That "C" standard spanner only tore up the metal without moving anything else on the damaged gun, so I gave up on the newer gun as it did not look like moving at all. Why so bloody tight? I don't really know, unless it was a deliberate strategy by the "Vega" factory to keep curious eyes from ever looking inside. I could even see where the aluminium had "set up" on the locating points (there are four of them each spaced at 90 degrees around the front bulkhead periphery) as the "C" spanner had bit hard into the metal, the factory assembly guy had taken two shots at it, shifting the spanner position each time, I could see the marks he left! I need that guy to load my supercharged pneumatic spearguns!
Propensity for Pressure Leaks
Anyway now I know why the RPS-3 guns can leak air at the reservoir ends; the rubber packing washers need to expand radially enough to seal on the inner barrel tube's outer wall by screwing in the small threaded aluminum rings that push against the rubber washers from the rear, causing the rubber washers to bulge outwards (and simultaneously inwards). However the threads on the periphery of the small aluminum rings are rather fine, too much force on them and they will strip out. The main screw threads used on the bulkheads, which are designed to take the gun's full chamber pressure, are much more substantial and being longer have a greater number of threads engaged with the matching internal threads on the outer tank.
Substitute Seals
I measured up all the seal seating diameters and the rubber washers can be replaced with "O" rings. The existing inner barrel to bulkhead seal at each end consists of two flat rubber packing washers located side by side, but the one nearest to the bulkhead wall can be replaced with an "O" ring of 11.80 ID x 2.65 SW (metric size R-10) which is a very good fit [from the Aldi "Workzone" yellow box set]. The second rubber washer is retained to provide a spacer between the new "O" ring seal and the thin steel washer and the threaded aluminum ring directly behind it. For the main bulkhead bosses (2) and the threaded connecting boss (1) to the rear grip a 7/8" ID x 3/32" SW "O" ring (imperial size R-15) [from the Aldi "Workzone" blue box set] can be substituted for the extremely poor quality rubber washers now used (3 rings). When using "O" rings on these bosses the large screw threads need to be temporarily covered up with vinyl tape to prevent the sharp edged threads at the annular grooved base of the threads cutting through the rings as they slide into the seating position. I learnt this the hard way as a brand new "O" ring was sliced in half before my eyes, then I took a much closer look to find out why. Rubber washers get squeezed up to seal when used as a packing type seal, so they can take a small cut and still seal OK, but "O" rings must be completely damage free to work. Their benefit is that they offer a dynamic seal, far superior to a rubber packing arrangement, provided the metal seating surfaces are smooth enough quality for "O" rings. Once converted to "O" ring seals the RPS-3 gun held pressure without a single bubble forming (in a bucket of water) for a couple of hours, something the rubber washer equipped RPS-3 gun could never do.
Failure to Operate
And now for why my "new" RPS-3 gun did not work before; water squirts out from under the rubber hose where it is tied with multi-cord wraps onto the inner barrel tube. I only discovered this fact by assembling the gun without the pressure chamber installed and then inserted the spear with my other hand pulling down on the muzzle to prevent any leaks occurring elsewhere and then I saw the water gush out. This in a "brand new" unused gun! I swapped the older gun's inner barrel for the new one and the gun now works, but still some water was found inside the pressure reservoir after pulling the gun down afterwards, so I think the cord wraps need retying as the "O" ring seals should be keeping water out. Hence the culprit must be the rubber hose in terms of how it clamps down onto the aluminium inner barrel tube. That is why the gun was never used!
Pressure/Force Considerations in RPS-3 seal area design
As the rubber packing arrangement with washers has to work in both directions, i.e. retain compressed air inside the reservoir all the time while keeping water at high hydrostatic pressure out of the reservoir when the gun was charged up to shoot they sealed the annular gaps at the bulkheads by designing them to be very small. This meant that the screw threads on the aluminum rings could also be small (with a fine pitch) as the force they worked against was low because of the small cross-sectional area of the annular gap that was exposed to high pressure air at the face of the rubber washers. Using "O" rings against the inner barrel instead of the rubber washers creates a larger "effective area" gap as air pressure faces the full section width of the "O" ring, which is much larger than the original parts gap. This does not matter at the front end of the reservoir as the pressure seals are located inside the pressure reservoir, but at the rear they are on the outside, just in front of the sear disc with the inner tooth. This seemed a mad idea at first, but they have done it that way to stop water from the charged gun penetrating forwards from the rear grip section's interior which will be at the same elevated pressure as the compressed air, and which is at slightly higher pressure as the spear is inserted which could overwhelm a rubber packing type seal. However hydropneumatic guns should be able to be opened up without depressurizing their air reservoirs, so to avoid the small screw threads on the rear aluminum ring failing due to the increased force now on the replacement "O" ring, a second "O" ring (11.20 ID x 2.65 SW R-09) needs to be installed as shown in the attached diagram, thus offering a two-way sealing system at this end of the pressure reservoir.
Why use "O" rings?
The benefit of using "O" rings is that they continually reseal when you dismantle and reassemble the gun, something that rubber washers are less likely to do, plus "O" rings require less effort in screwing the parts together as there is no crushing required of rubber washers. Why did the Soviets use rubber washers instead of "O" rings? Well Soviet "O" rings for non-military use were crap and were just not up to the job.
