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PostPosted: Mon Jul 20, 2015 2:41 pm 
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Location: Vancouver
So I've got this modded QB78 carbine. And the other day a 13ci Ninja tank arrived from Scoped & Ammo, so I'll be setting it up as a regulated bottle gun. It's been modified in a few ways, most obviously with a Crosman pistol grip, but with the remaining 14" barrel it's silly-long for a pistol and just right for a carbine, so I added a 1" aluminum pipe stock. Nice and comfortable. And there's an 8-shot manually advanced repeater magazine in place which is kind of fun. It's got a Foster fill fitting installed in a brass adapter held in place by the native screw-on cylinder nut, the nipple set at 45 degrees downward. It'll hold 2,000psi as it is, but I'd like a lot more shots plus the regulator and would prefer to run at the stock Ninja pressure of 850psi, using the whole cylinder as a plenum. I might choke the cylinder at some point (as I did with the much smaller plenum space in my now-regulated HiPAC) with a plastic pipe filler, if it proves the vast plenum of the QB78's main cylinder is too much for the relatively low power I'll be running it at. But for now my problem is in how to attach the Ninja bottle in as compact and light weight a manner as possible.

Bob (rsterne) has graciously forwarded a couple of versions of his reversed/dropped tank block for this rifle to Rick (rrdstarr) for when he's ready to machine some. If I went with one of those I'd go with the one which uses the built-in threading and nut to capture the top end of the drop block. But when I ordered the Ninja bottle it occurred to me that I might also be able to make a smaller aluminum block work, using a female Foster QC fitting mounted in the top of that at a 45 degree backward angle and spaced such that it could snap into place, then the bottle could be secured using a fitted shim between bottle and cylinder then a couple of bands to firmly lock it into place on the cylinder eliminating any stress on the QC fitting. Like this:

Image

In my Paint sketch over a photo I've pasted in a female Foster QC (hasn't arrived yet) then drawn in the rough shape of the aluminum drop block. The block would be threaded for the 1/8 BSP QC connector up top, milled out at an angle such that there is no interference with the QC sleeve when it has to come off for maintenance. The bottom rear face would be drilled and threaded to match the Ninja bottle, and hopefully timed such that the gauge is on the right, fill nipple on the left, to keep anything delicate from facing down in case of having to rest on something like a branch or fence. The red line is a hole bored through for air to go up through the QC fitting and into the cylinder. Seems simple enough. A couple of close tolerance bands bolting the bottle to the cylinder against a form-fitted full-length plastic shim would keep the bottle firmly in place and provide a good forestock, perhaps with a covering of thin neoprene. My neighbouring outdoor fabrics store has loads of small roll-ends of that sort of thing, like diving suit material.

My question is this: will a Foster QC fitting be okay for long-term use in such a setup? It'd be under 850psi pressure 24/7. I've got the same fitting on my FX pump and it seals perfectly (actually put in a new seal I made of hard urethane, as the original got blown out somehow) but that's just for a few minutes at a time. Anyone used a QC like this? Is this the sort of thing paintball bottle guns use and it's okay for long term storage? I'd happily go with the more 'permanent' sort of bulkier drop block like Bob is using on a couple of his airguns, but if I went with something like this instead there's the convenience of still being able to use the airgun if the Ninja bottle ever fails. Just pop the bottle off and fill the cylinder directly. I could even get a Lane regulator for the QB and shoot it without a bottle, for a much more compact little carbine. Of course I'll probably love the bottle volume too much to go back... but flexibility seems a good idea, and the angled brass nipple adapter is already there and leak-free so why waste it? Thoughts?


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PostPosted: Mon Jul 20, 2015 3:04 pm 
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Location: Coalmont BC
No reason why it won't work.... RE the idea of using a Lane regulator, though, remember you are restricted to not more than CO2 pressure on a hot day (1800 psi @ 120*F) on the front end of the tube.... One quick look at the thickness of the flange on the end of the CO2 retaining nut will tell you why.... The same thing goes for the valve retaining screws, and the tube itself.... DON'T use HPA on a QB tube, valve and front nut without a 1.8K burst disc to protect it....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Mon Jul 20, 2015 3:18 pm 
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Location: Vancouver
Thanks Bob! That's reassuring, if you think the connector should be able to hole in the long run. I may have to upgrade the seal at some time but at worst maybe it'll leak under constant pressure then I'll know. I'd tested the QB at 2,000psi (while wearing eye and ear protection and never aligning myself with the tube sides nor the cylinder front end during filling) figuring that the way the valve was secured and the brass plug is quite deep with two O-rings it would probably hold okay. In later tests I never went higher than 1,600psi, figuring I didn't want to push my luck. But I'll probably never use the thing again without the bottle. And 850psi will be safe enough. Even if I dial down the spring resistance in the Ninja to 1,000psi or so some day it won't be unsafe. So no need to install a burst disc in the QB, but thanks for pointing that out. Guess I was being an idiot taking the risk at 2,000psi even just once. Curiousity, don't you know... but if I did ever wish to run 2,000psi with just the QB tube I suppose pinning the brass plug with a high grade bolt shaft or a tungsten rod would probably do the trick.


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PostPosted: Mon Jul 20, 2015 7:05 pm 
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Location: Coalmont BC
The problem is seldom whether something will work once, but what happens when you repeatedly push it beyond 50% of the yield strength of the material, which causes it to fatigue, crack, and eventually fail.... That is why a 3:1 safety margin is used, to avoid fatigue issues.... You are correct that 850 psi is not an issue, PROVIDED you still have the 1.8K burst disc in the regulator on the downstream side.... Otherwise, a regulator failure could put full tank pressure into your QB....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Mon Jul 20, 2015 7:22 pm 
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Joined: Tue Oct 13, 2009 1:27 am
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Location: Vancouver
Well there you go again, being helpful while raising another question. I've seen you and others in various forums mention this sort of proviso; that so long as the burst disc remains on the downstream side... I haven't modified my Ninja bottle nor regulator in any way. Is there a way in which a person might move the burst disc to the upstream side of the regulator in the bottle? Drill and thread a special hole for it perhaps? Joking of course, but I really don't get why this needs to be specified, and not getting it makes me a bit nervous, like perhaps there is something about the Ninja bottle, maybe a couple of different models...

So of course now I go to my 13ci Ninja bottle and look into this puzzle. And I find that there are two 10-32 set screws. So I loosen those, which allows unscrewing the end of the thing. I unscrew and find out that in fact the regulator is part of that unit. DOWNSTREAM from the regulator! Have I completely messed up and ordered the wrong sort of tank for this conversion? Is there a Ninja bottle with the 1.8kpsi burst disc (and mine has a 5k burst disc as well, very slightly further back from the end than the 1.8k) somehow installed on the downstream side of the regulator instead of the upstream side as on the one Eric sold me? Colour me thoroughly confused and now a bit worried thanks to your cautionary note.


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PostPosted: Mon Jul 20, 2015 8:41 pm 
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Location: Coalmont BC
Referring to your photo, the burst disc on the top is the downstream one (closer to the outlet) and the one on the bottom is the upstream one.... The upstream one is a 5K and prevents you from filling the bottle past it's hydrotest pressure of (5/3 x 3000) = 5000 psi.... The downstream one should be a 1.8K on an 850-1200 psi output regulator, but sometimes guys replace it with a 3K, particularly if they want to run 1600 psi or more.... That's OK unless the regulator fails, and bypasses whatever pressure is in the tank into the gun.... If you look inside the body of the regulator, you will find that the upper (downstream) burst disc vents into the bottom of the chamber that the piston runs in (between the small O-ring and the HP seat).... which operates at the output (downstream) pressure.... The bottom burst disc is on the same plane as the male Foster fitting and the gauge, and all three are on the other side of the HP seat, and hence at tank pressure....

HTH's....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Mon Jul 20, 2015 10:50 pm 
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Joined: Tue Oct 13, 2009 1:27 am
Posts: 2514
Location: Vancouver
Okay, dense guy finally getting what you're saying. Opened it up again and looked more carefully and see that there is a tiny hole drilled in from the 1.8k disc visible in there while the higher pressure one is behind the 1/8" or so of aluminum. I was letting myself get too hung up on the 'upstream/downstream' imagery and thinking it'd have to be a big, obvious location difference, but of course the flow doesn't have to be so linear as that. Makes perfect sense now, thanks for clarifying. And no, I wouldn't be operating it at anything near that kind of pressure so no risk of my swapping out burst discs. I'm not all that keen on the high power stuff.


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