need advice on a AR build

[renowntwo]

Hi all and thanks in advice from all those more knowledgeable people. I'm considering a .25 cal build and I'm wondering about the porting sizes.
The AR will be built on a qb, I'm undecided which qb. What will be the advantage in using the qb78 over qb79 or vice versa?
The air source will be a ninja tank at what pressure should I run? I want to shoot in the 850 fps range with let's jsb if they have the accuracy.
Would a RVA help?
What about the hammer and spring, would I've do anything to them or will the stock parts work?
Will be asking more question later also if there any thread please can you post them.

[UCChris]

Using the 78 will allow for a huge increase in plenum volume.

[rsterne]

I would definitely base your build on the QB78 tube to get the extra plenum volume downstream of the regulator.... You will find that 75% boresize ports (ie 3/16") are large enough for the 25 gr. JSB Kings to over 50 FPE using 1800 psi.... You can get a slight gain in efficiency by using 7/32" ports, but that will require an oblong barrel port so that the width stays at 3/16" to avoid pellet damage.... but you can't use the extra power the large ports are capable of with a 25 gr. pellet.... You will need an RVA, or have to play with cutting or shimming a hammer spring, in order to reach the best efficiency on any regulated gun, no different than unregulated.... I haven't built a .25 cal QB, so I can't answer your question about the hammer and spring, but my gut feel is that you can make the stock items work.... I use a QB hammer spring in my 2560 HPA which gets 52 FPE on 1800 psi....

Note that in order to use 1800 psi safely in a QB, you should consider replacing the tube with one of known quality steel.... You will also have to make sure that the valve and tank block mounting is secure enough to stand 3000 psi, should the regulator fail.... If you do not wish to have to worry about that, then you should stay with a maximum of 1500 psi so that the regulator can use the original 1.8K burst disc to insure that the gun never sees more pressure than it could with CO2 on a hot day....

Bob

[UCChris]

Just to add to Bob's advice.

Jason has had reliability problems with the stock burst disk bursting at pressures well below the 1800 psi mark, so he fits a custom 2k burst disk. You may want to talk to him to see how he does it.

He also uses aftermarket screws to secure the tank block as far as I can tell on mine. Hopefully he will be along soon to expound on my amateur assessment...

[rsterne]

Upgrading the tank block screws is a good idea, but they are too close to the end of the tube, and can rip out.... You need to do something like this....




It is also critical to insure that the valve is sitting right against the steel backing block so that the screws in that block take their share of the end force on the valve, sometimes there is a gap between that must be shimmed if you are using more than ~1500 psi.... Going too high in pressure brings the tube itself into question, as it is thinner than what is used in a Disco (0.059" vs. 0.065", and of unknown tensile strength) and was not necessarily designed for pressures over that of CO2....

I would be very interested in where Jason found 2K burst discs....

Bob

[Raizer]

My 79 has a 78 tube, 1300psi JDS tank, BSA .25 barrel and a one off custom made valve, it currently puts out 40fpe with JSB Kings.
That's using the stock hammer spring preloaded up as much as possible while still letting the bolt open far enough to cock/load pellets (it's cock on open modded)

Once I manage to get hold of some Belleville washers I'll be trying it on 1500-1550psi.

[nervoustrigger]

Regarding the burst disks, yes I've had more than one of the 1.8k disks on the Ninja tanks rupture at only 1400psi.  In case you're wondering, I know the regulators were not creeping, having tested them with a 1% gauge for over 24 hours and multiple open/close cycles.

I used a hollow punch to knock out some aluminum disks from the side wall of a soda can (0.005" thick), and screwed the ruptured 1.8k disk down onto it.  According to my notes, the aperture on the 1.8k disk measured 0.153" diameter (if the hole is a different size, that will affect the burst pressure).  I then tested a couple of samples and found that they burst at too low of a pressure.  I went hunting a slightly thicker material but could not find anything suitable.  I decided to try laminating clear packing tape and found that a single layer yielded the desired result.  Concerned that the tape might be cut by the applied pressure against the aperture of the burst disk, I intuitively placed the taped side away from the burst disk.  Testing multiple samples gave a consistent burst pressure of between 2000 and 2100psi.

I feel compelled to say that high pressure air is nothing to take lightly.  If something goes wrong, it can be deadly so be absolutely sure you know what you are doing if you decide to make and validate your own burst disks.  Do not assume that you can go cut a disk from a soda can and expect it to act as a 2000psi burst disk.  The alloy, thickness, and aperture size all play an important role.

[rsterne]

Custom making your own burst discs is a risky proposition, for sure.... One of those things that CAN be done but do you really want to take the risk?.... Do I understand you were using one layer of pop can PLUS a layer of packing tape?.... The aperture diameter in the ruptured disc is critical, as you state, not only the thickness and tensile strength of the membrane material.... A source of accurate material could be brass shim stock, which should be available in 0.001" increments, and consistent in thickness.... although it could still vary in tensile strength from batch to batch....

Bob

[nervoustrigger]

Risky, yes.  That's why I validate multiple samples to failure each and every time I match a new diaphragm to the body of a ruptured burst disk.  I make no assumptions that anything is a perfect match to the last one I tested.  That puts risk on a pretty even keel with other things we commonly do when working with HPA.  Heck, having seen plenty of examples of material and process variations in my years of dealing with production and QA issues, I would say I have more confidence in my burst disks than the factory stock ones made to some unpublicized QA standard.

To answer your question about the particular combination of parts used to make one of my 2000psi burst disks, it was indeed one 0.005" disc of aluminum from a soda can laminated with one layer of packing tape (0.0026" thick), fitted against a bust disk body having an aperture of 0.153".  Again, though, I would not assume this recipe to work every time so anyone attempting to roll their own should test to failure to confirm.

Good point about the brass shim stock.  I was thinking along similar lines before coming upon a solution with common household materials.

[renowntwo]

Thanks everyone for your help. Another question
What should the shot count be?
Would barrel length play a part ? If so, what is the minimum length?
Would the crosman transport seal for 22xxx AR still work or will I need to use else ?

[rsterne]

I get ~ 30 shots at 52 FPE on 1800 psi in .25 cal from a 17 CI tank in my 2560.... no reason a QB based HPA shouldn't be similar.... At 850 fps (40 FPE) and using a 13 CI tank at 1500 psi, you should be able to get close to that.... The longer the barrel the more power or efficiency, I would use about a 24-25", not less than 20", for that power level.... The Crosman 22XX transfer port works will in a QB/AR build....

Bob

[renowntwo]

Thanks again. More question to come  LOL.