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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Sun Sep 17, 2017 7:41 pm 
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Location: Coalmont BC
The limiting factor, from what I can see, is the original QB threaded cap that retains the block your fittings are screwed into.... One of my friends in the USA who has access to a computer program for running stress analysis (FEA) concluded that the thin web on that cap limits out at about 1800 psi MSWP, which coincides with the maximum pressure of CO2 at 120*F.... I would not run more pressure than that, as an absolute maximum.... Using a 1.8K burst disc on the output side of the regulator to protect a QB in the event of a regulator failure remains a prudent idea.... If unregulated, I would not fill beyond 1800 psi....

Bob

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Mon Sep 18, 2017 12:05 am 
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OK thanks Bob, good information, and appreciated.

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Mon Sep 18, 2017 10:20 pm 
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Bear in mind the 1800 Bob mentions is a max pressure. Unregulated set for 1600 or so to be safe. Don't want no kablooies. :mrgreen:

-D.S.

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Tue Sep 19, 2017 1:11 am 
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Hi Doc! Hope you had a good summer! Thanks, fair enough, though Bob has been doing some planning on building a double 88g gun on the GTA forum and it looks like he plans to go to 1800psi.
It's been pretty quiet on this forum lately! Will be nice to see guys start doing more projects once winter kicks in.
Have a great week!

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Tue Sep 19, 2017 9:29 am 
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Providing ALL issues are addressed, including valve fixture (making sure it is sitting against the retaining ring), the front retaining ring strength, etc.etc.etc.... I don't see a problem with using a MSWP that aligns with what CO2 can do on a hot day.... We haven't heard of even one QB7X failing on CO2 after being left in a hot car, or in direct sun.... On a regulated HPA conversion, I would use a 1.8K burst disc to prevent the gun in the event of regulator failure.... Using a hand pump, or with EXTREME care in filling (eg. using a regulator to fill, or a slow-fill valve), I would stick rigorously to 1800 psi or less.... JMO....

Bob

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Tue Sep 19, 2017 6:11 pm 
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Location: P.G. B.C.
Not likely to happen this late in the year, however much depends on location. A friend who lives near the border, drys his bow staves in his car in the Summer time, as the inside temp hits 150F. He has to lay the stave blanks on towels to absorb the water.

150F inside a hot car - considerably higher than 120F. I would not leave a CO2 or compressed air rifle in a car with windows up.

Bet your auto insurance would no cover damages if it exploded.

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Tue Sep 19, 2017 8:34 pm 
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Well turned down the power and shot this string from 1800 to around 1050psi. I used 15.89 jsb and shot 29 shots with an average of 850.5fps. The fpe/ci figured out at 1.29, with an estimated air volume of 182cc, so less efficient than last time with the 21 gr (over 1.5fpe/ci) which makes me curious. Does having too much free flight on the hammer cause less efficiency? I am not using an ssg or bstaley o-ring buffer. Anyway, I'm still happy with the nice curve and shot count! 26 good shots, at least acceptable to me.
Here is the string:
830, 825, 843, 838, 854, 857, 841, 857, 868, 874, 857, 863, 877, 866, 871, 863, 863, 871, 877, 854, 868, 851, 833, 838, 838, 830, 820, 820, 817.
A little bouncy up and down, maybe need some more polishing?
Anyway I'm happy for now, looks like a good power setting for me. Maybe next I'll have to try a few orings in the back to see if I get more efficiency that way. Need to enjoy it for awhile as it is first though☺️.

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Tue Sep 19, 2017 8:38 pm 
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Well that was kind of my point.... You can bet that somebody, somewhere, has left a QB inside a car at 150*F.... and I would equally bet that had even ONE failed, we would have heard all about it.... To put this in perspective, a properly filled CO2 tank, filled to 100% of its rated capacity by weight, would be close to 3000 psi at 150*F, and by then the 3K burst disc would likely have failed.... If the tank was only 75% full, however, the pressure would be under 2000 psi at the same temperature.... Now don't get me wrong, I'm not suggesting that you store any CO2 gun at 120*F, let alone expose it to 150*F.... However, it is to look after those circumstances that a gun is designed with a large safety margin.... ASME rules for pressure vessels require a 3.5:1 safety margin to burst (ie based on the UTS of the material) above the Maximum Safe Working Pressure (MSWP).... Any vessel over 2" diameter must be Hydrotested on a regular schedule to 5/3 of the MSWP.... For CO2 tanks, they are rated at 1800 psi MSWP, and Hydrotested to 3000 psi, and fitted with a 3K burst disc.... However, the burst pressure for that 1800 psi rating must be at least 6300 psi, giving over a 2:1 safety margin at the point the burst disc lets go.... or inside that 150*F car interior....

I'm not telling everyone to use 1800 psi as their fill pressure on a QB, because some of them don't have the valve sitting against the steel backing block.... The valve only has one mounting screw, but the backing block has two more, and they should ALL be taking the load.... One of those three screws holds down the receiver, so you should never pull the barrel off a QB with gas in it for that reason.... Computer FEA analysis has shown that the flange on the front retaining nut on the tube is the weakest point on a QB78, and depending on the quality of the steel we assume it has a MSWP of about 1700-2000 psi.... The two screws that retain the tank block on a QB79 have an even lower MSWP, but again, we have never seen a failure on CO2.... I do know of one tank block that pulled out by shearing the screws at 2600 psi when the regulator failed, but the action was out of the gun, and the two additional stock screws weren't in place.... Whether they would have prevented the failure is doubtful, but again, we don't see failures in QB79s on CO2....

When I am converting a QB to HPA, I always check the valve to make sure it is against the backing block, and insert a shim if necessary, so all 3 screws share the load.... If I am using more than 1100-1200 psi on a QB79, I install an extra shear pin in the tank block for added safety.... Once I address these obvious, and well known, potential weakest points, on a regulated QB I still use a 1.8K burst disc, which limits my setpoint to less than 1500-1600 psi so that I won't blow the disc.... On a properly inspected and prepared QB78, I would not personally have a problem filling to 1800 psi, but I would not go past that without special modifications to valve and end plug mounting.... The tube itself, assuming mild steel tubing, could fail at as low as 8000 psi, which means that since we don't know what it is made of, 2300 psi would be a safe MSWP.... Having said that, the virtually identical tube on an XS-60c has survived testing to 10,000 psi without failing, although other components failed well before that....

Bob

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Airsonal;
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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Tue Oct 03, 2017 9:45 pm 
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Well I have been having some fun with my Qb78, finally got a chance to do some 50 yard shooting, but it was terrible windy, around 20mph with gusts even higher, so not a good day for checking accuracy lol. I set up with the wind blowing from behind me to the right so at least didn't have a total cross wind. I had the gun set to shooting 21gr H&N averaging in the 840'sfps. I managed to get three 4shot groups under an inch and four 5 shot groups at 1.5". Still, wasn't any way to tell how accurate the gun really is. Will have to try again another day!

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Tue Oct 03, 2017 10:29 pm 
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Yesterday and today decided to try out the bstaley oring mod. With the two tanks and only a free flying hammer I had gotten up to 1.78fpe, which was amazing, but with just the one tank I seemed to be not getting more than 1.33fpe/ci and mostly 1.2 something.
I put a 3/4" oring in and it seemed very erratic for some reason, and efficiency didn't improve, in fact it seemed to be wasting more air. I switched to a thicker oring this evening and using 14.3gr cphp, got 24 shots avg 898fps and over 1.5fp/ci.
The weird thing with that string was that it started over 900fps and near the peak velocity and slowly started dropping, I realized I'd have to turn out the rva a bit to get a better curve so I turned it out half a turn and the velocity jumped back up! I did this 3 times, every time the velocity started dropping I'd turn it out half a turn and the velocity would jump up again, instead of going down. I also noticed in initial testing that putting more tension on the spring eventually caused a decrease rather than increase in velocity. Does anyone know what causes this? It would get louder but slower[emoji53].
Anyway that is my mystery for the day lol[emoji4].
The last string I did was 23 shots over only around 425psi with 21.14gr H&N, averaging in the low 700's, and ended up with an efficiency of 1.62fpe/ci. Big improvement! Only downside is the low fps with the heavier pellet. I'll have to turn the rva in a bit more and see if the fps goes up and the efficiency is maintained or tanks. Another day!

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Wed Oct 04, 2017 11:07 am 
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Can sure see the advantage of regulating and a 3000psi bottle! 13ci tank regulated to 1500psi would give 100 shots or more at 25fpe. Only problem is they are 2" wide instead of 1 3/8", which are makes for heavier bulkier front end. Would have to cut the stock right off and make a bracket at the back to hold the bottle as others have done. We need some skinny 8ci regulated bottles with 3000psi capacity! That'd give 70 shots instead of 22 or 23[emoji5]

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Wed Oct 04, 2017 11:53 am 
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wesb2007 wrote:
Can sure see the advantage of regulating and a 3000psi bottle! 13ci tank regulated to 1500psi would give 100 shots or more at 25fpe. Only problem is they are 2" wide instead of 1 3/8", which are makes for heavier bulkier front end. Would have to cut the stock right off and make a bracket at the back to hold the bottle as others have done. We need some skinny 8ci regulated bottles with 3000psi capacity! That'd give 70 shots instead of 22 or 23[emoji5]

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13ci tank isn't much deference for 2" to 1 3/8 ..your not going to feel much deference in the front end..
now im a little confused were are you coming up with these numbers that a reg set up for 1500 psi otput
will give you more shots... :?
yes you will get more power=FPS with that reg setup not more shots per say...

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Wed Oct 04, 2017 12:43 pm 
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Ace wrote:
wesb2007 wrote:
Can sure see the advantage of regulating and a 3000psi bottle! 13ci tank regulated to 1500psi would give 100 shots or more at 25fpe. Only problem is they are 2" wide instead of 1 3/8", which are makes for heavier bulkier front end. Would have to cut the stock right off and make a bracket at the back to hold the bottle as others have done. We need some skinny 8ci regulated bottles with 3000psi capacity! That'd give 70 shots instead of 22 or 23[emoji5]

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13ci tank isn't much deference for 2" to 1 3/8 ..your not going to feel much deference in the front end..
now im a little confused were are you coming up with these numbers that a reg set up for 1500 psi otput
will give you more shots... :?
yes you will get more power=FPS with that reg setup not more shots per say...

You are probably right Ace, about not feeling that much difference. I looked up the weight of the 13ci bottle with regulator and it is only about 4 more ounces than the 88g with adapter, so not a big difference, and the width probably wouldn't be that bad either. The shot count jump is comparing a 88g on hpa with a max fill pressure of 1800psi vs a same volume regulated tank at 1500psi regulation with a 3000psi fill. I chose 1500psi as I thought that was probably the sweet spot for getting powerful shots in the 25fpe range. If the efficiency remained the same as what I just got with my unregulated gun, it would give around 70 shots according to Sikes efficiency calculator.

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Wed Oct 04, 2017 12:55 pm 
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I feel I need to apologize for the previous 1.78fpe/ci efficiency number as well as the 1.5 something efficiency I reported further up the thread a week or two ago. I was using the gauge on the gun which does not seem to always be accurate, compared to my pump, so I have started using my pump's gauge to measure pressure at start and end lately, which has dropped the efficiency some, but still seems a bit higher than others have posted, so I'm a bit sheepish about them as well, though I'm not sure what I am measuring wrong, other than I should be giving a low guess as well as what I think the gauge is saying. Even inflating my gun volume to 185cc, guessing a lower estimate on the end pressure, I'm still at a minimum of 1.5fpe/ci on the last string, which is still pretty amazing I think! Anyway, exaggeration is really annoying, and most of all in myself, so I am trying to be less optimistic and more conservative in my postings from now on.

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 Post subject: Re: Qb78 deluxe hpa
PostPosted: Sat Oct 14, 2017 3:47 pm 
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Well, got out and shot some 50 yard targets this morning. Tried a couple different scopes. Thought maybe my cheap 6-24x50 wasn't holding zero, so started with my 3-9x40 Simmons. Was getting around 1" groups with it, but nothing great. I brought 3 tins of pellets with me this time, all made by jsb. In size order, Cometa 14.3gr, Jsb jumbo exact 15.89, and Jsb 18.1 heavy, all supposed to be 5.52mm.
Once I switched back to the big scope, I was happy to get better groups. In the one picture is 3 different pellet groups, I'm using different points of aim. The heavies were 5 shots 3/4"(edit:closer to 7/8"), the 15.89 were 5 shots 5/8" and the Cometa surprised me and were 8 shots at 1/2"(edit:actually 3/8"!) excluding one flyer that happened because these short pellets 14gr pellets seem over balanced and flip up easily and I jammed it a bit and bent the skirt and like a dummy chose to shoot it anyway and ruined my group! That was the 7th shot, then I finished it with the 8th right in the same hole area as the others. There is another picture of a 3 shot group with the cometa pellets and repeat pictures of the 18 and 15 gr.
Great morning! - 17 Celsius earlier this morning but quickly warmed up to - 7, and now it's plus 9 Celsius, but the wind came up as I was shooting.
Still never had time to zero with best pellet, next time! Good way to spend a morning, even got to shoot some splatter burst discs[emoji2] ImageImageImageImageImageImageImageImage

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