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Jaxom · Senior Member

Bob,

That valve you made, is that stainless or aluminium? What pressure ratings do you think it could handle?

The second project I would like to do would be a disco based. But in .25 caliber. I wanna push the psi higher, but then, unlike my 2300, install a power adjuster.

aircube · Veteran Member

Hey Bob,

Why did you choose the Disco valve over the Boss Max Flow bulk valve?

rsterne · Moderator

I modified a Disco valve, which is aluminum.... also the guage port is aluminum.... I figure if it works for the Disco at 2000 psi it's not a problem, as long as you watch the wall thickness.... I would never use Stainless because it's a bear to work.... The Disco guage has the "red zone" starting at about 2400-2500 psi on the valve, so I wouldn't worry about it up to there.... Don't forget, if the valve fails (as long as it's in the gun) all that will happen is that it will vent and scare the heck out of you.... the bits will be retained by the main tube.... Any bored out aluminum valves I have heard about failing just cracked and leaked.... Cool

One thing to keep in mind.... most PCP guns that run 3000 psi don't get their maximum velocity at that pressure.... They either have a self-regulating shot curve which peaks quite a bit lower, or they run a regulator to drop the pressure.... Like I said before, cranking up the pressure will usually reduce the velocity without leaning HARD on the hammer spring.... and there goes your shot count.... Rolling Eyes

I used the Disco valve for two reasons.... it's easily available in Canada.... and I like the way it fastens into the tube, with three 8-32 pan head screws with the heads in shear.... simple and strong.... I would think a Boss Max-Flow Valve would work fine in this application, as long as they left a thick enough wall to stand the pressure of HPA.... from the cut-away on the webiste it looks like about 1/16" wall.... I don't see any provision for a stronger mounting, however.... It might have one, it's just not visible in the photos.... There is NO WAY I would trust the single 6-40 screw that mounts a 22XX valve to withstand the pressures of HPA.... Shocked

Bob

Jaxom · Senior Member

I understand the concepts behind pressures, as you have explained them before. I don't have a chrono, or any ag's to experiment with. What would be a good pressure to shoot for when concidering converting a disco to .25 caliber?

rsterne · Moderator

I would use the standard 2000 psi fill if you are using the standard reservoir.... If you are using a CO2 (paintball) tank, they are usually rated at 1800 psi, so no choice there.... If you are using a tank and regulator like those from JDS Airman, you have a choice of 1400, 1600, 1800, or 2000 psi.... Cool

What I have found is that without changing the hammer spring, the velocity (in .22 cal) tends to peak at around 1600 to 1800 psi, depending on spring preload.... When you crank the tension to the maximum, you will notice significantly increased trigger pull weight, so my guess is that using a stronger hammer spring might be problematic.... Shocked

I don't know what the change to .25 caliber will do to the shape of the shot string.... According to Straight Shooters in their "Our Take" on the Disco, in .177 cal the fastest shots were the first 10 (indicating that the hammer spring is too strong), while in .22 cal the fastest shots were shots 10-20 (consistent with my results)....

http://www.straightshooters.com/ourtake/ottestdiscovery.html

I would theorize that in .177, the short, sharp pulse at the beginning of the shot string (when the valve is being closed quickly by the 2000 psi air pressure) is capable of driving the light pellet faster than the longer (but lower pressure and higher volume) pulse later in the shot string.... while in .22 cal the stock hammer setting produces a well balanced shot string (which peaks at about 1700 psi).... Extrapolating this to .25 cal might mean that even lower pressures (but more volume escaping) would be the "sweet spot".... or maybe not.... If I had to guess, a .25 cal would likely benefit from a larger volume valve with bigger ports, transfer port, and barrel port.... Rolling Eyes

I would assume that you want maximum power (otherwise, why go for a .25 cal).... so I'd start with a stock Disco, change the barrel, add a velocity adjuster, and get yourself a Chrony.... It is realistically the ONLY way to find out what you have and what any changes you make are doing.... Wink

Bob

Jaxom · Senior Member

Good advice Bob, Thanks a heap!

yourdaddyjoe · Supporting Member

True, True Bob... on the porting.

I know when I have gone toooo far on porting & not enough hammer spring tension... I get a nasty machine gun bounce... oooops... there goes a tank of air.... It would be fun if I had an Auto loader & hooked into the SCUBA tank... Twisted Evil

Jaxom · Senior Member

Hope ya don't mind the questions is all. Sometimes I admit to repeating myself. I had at one time concidered that I would be doing two 22xx projects. Well, in a way the discovery is in the same family from everying I've heard discussed here.

To do another "sniper" type 22xx would certainly be fun. But I also want it to do a bit more then a .22 would. That's why I'm thinking of going with a .25 caliber.

I just didn't want to fall for the old trap of "more is better" when it comes to pressure. I'd rather my second project be a harder hitting tac driver then some over blown hpa/gas guzzler.

I have a few other questions about what I'm thinking, but I don't want to thread jack this post any more then I already have. So I'm going to start a seperate thread...

Jax

rsterne · Moderator

Got a few parts I was waiting for and had a chance to finish assembling Version 2 of the 22XX PCP....

This Version has a Leapers 3-9 x 40 AO Scope and is based on a 2250 tube using the parts in the post above.... With no change in the velocity adjuster setting from the previous version I picked up 35-40 fps with the bored out valve.... There may have been some advantage to the additional volume between the valve and the guage adapter and the streamlining on the forward portion of the valve, but it is impossible to separate the individual gains.... In any case, it was well worth the trouble.... Wink

What was even more interesting is that with the larger valve volume, still at the same 1600 psi, I am now able to lean on the velocity adjuster (ie more hammer spring tension) and gain even more performance....

This chart shows the original velocity curve (using JSB Express and stock internal valve volume) and you can see that it tops out at 890 fps when the adjuster is still 3 turns out from coil bind.... With the larger valve, I can now gain velocity all the way to coil bind on the Disco hammer spring.... The total gain is ~100 fps and 22%-25% more energy over the previous version.... Shocked

This latest version produced the following results at maximum hammer spring tension:

14.3 gr. JSB Express; 988 fps, 31.0 FPE
15.9 gr. JSB Exact; 954 fps; 32.1 FPE
18.0 gr. JSB Heavy; 916 fps, 33.5 FPE
21.6 gr. Kodiak/Barracuda; 842 fps, 34.1 FPE

I still have to shoot a complete tank (from 3000 down to 1600 psi) to find out how many shots I lost.... but I'm guessing it will shoot about 50 shots at the maximum setting.... At 3 turns out on the adjuster with the stock volume valve, I was getting 70 good shots averaging 25-27 FPE depending on pellets.... Cool

I am very pleased with the results.... The gun weighs just 6 lb. 2 oz. c/w tank and scope, and it balances beautifully (right between the trigger and the internal guage).... I'm still going to fine tune it to get the best balance between power and efficiency and I'll report back with the efficiency numbers (FPE/cu.in.Air) later.... Twisted Evil

Bob

sniper · Silver Status Member

bob,

at joe's mention. please write the book already! PCP'S FOR DUMMY'S. Laughing

rsterne · Moderator

I did some testing today using JSB Express 14.3 gr. pellets to determine the range of adjustment provided by the velocity adjuster, the number of shots per fill at each setting, and the overall efficiency.... The efficiency is based on the thoughts in this thread: http://airgunhome.com/agforum/viewtopic.php?t=3448.... As discussed in that thread, I have turned the formula upside down so that the numbers come out as FPE/cu.in. of air used (at one atmosphere).... Here are the results:

The first thing to notice is that once you go below about 620 fps the velocity starts to fall off rapidly, the shot count increases steeply, and the efficiency DROPS.... This gun has peak efficiency at that point, and will deliver about 190 shots per fill at 12.2 FPE.... It is also very quiet at that setting.... Note that this gun can be set up for under 500 fps and it is about as loud as someone clapping their hands.... Shocked

The next thing to notice is that about 957 fps the velocity curve starts to flatten out, and the efficiency starts to drop quickly again.... and the guns gets extremely loud.... At that point, the gun is delivering about 50 shots at 29.1 FPE.... Cool

Between these two points, the velocity gain per turn of the adjuster is nearly linear.... with the efficiency decreasing as the velocity increases.... Winding the hammer spring tension in the last two turns only increases the velocity about 30 fps while the shot count drops from 50 shots to 40.... As the gun is currently configured, 2 turns out from coil bind on the velocity adjuster seems to be the optimum for hunting efficiency.... Wink

At that setting, I am getting 50 shots per fill, shooting down from 3000 psi to 1600 psi (the regulator pressure) and it takes 150 pumps (3 pumps per shot) to refill to 3000 psi.... I also tested two heavier pellets.... the results are below:

JSB Express 14.3 gr; 957 fps, 29.1 FPE
JSB Exact 15.9 gr; 915 fps, 29.6 FPE
JSB Heavy 18.0 gr; 867 fps, 30.1 FPE

I think that 30 FPE and 50 shots per fill are fabulous performance from a 22XX, especially on only a 13 cu.in. tank.... Combine that with the ability to adjust it down to a very quiet 12 FPE and get 200 shots per fill with just 6 turns on a screw, and it's pretty flexible as well.... Twisted Evil

Bob

fritz · Silver Status Member

So... how much for it? 50 shots at 30fpe with a 13cu tank... at 1600psi... WOW.

sniper · Silver Status Member

i say again,

YouAreAGod

i'd be curious to hear what bob sounds like talking in his sleep with math formulas! Laughing

rsterne · Moderator

I have heard from some forum members wishing to duplicate this 22XX PCP project.... I want to make SURE that everyone understands the dangers associated with asking parts to do things they were never intended for.... These notes also would apply to modifying a QBxx to run on HPA.... NOTE, I AM NOT RECOMMENDING THE USE OF HPA IN A GUN DESIGNED FOR CO2.... YOU DO SO AT YOUR OWN RISK.... Exclamation

First off, the 22XX main tube was never intended to hold pressure.... All the CO2 pressure is inside the powerlet and the valve ONLY.... In a QB78, since there is a factory bulk fill ket available, and in the QB79 you simply screw on an external CO2 tank, the main tube is obviously considered "safe" for CO2 pressures, assuming the gun/tank is NOT overfilled.... People (me included) assume that a Crosman 22XX main tube is at least as good as that on a QBXX.... but that assumption could possibly result in injury or death.... Here is the CO2 pressure chart:

http://www.warpig.com/paintball/technical/gasses/co2pv.gif

As you can see, although "normal" CO2 pressure is considered to be about 850 psi, on a hot day it can reach about 1800 psi.... CO2 tanks are rated at 1800 psi for this reason.... Although it is NOT guaranteed, if you protect your gun with a 1.8K burst disc you should be able to sleep at night.... The problem is that those burst discs are plus or minus 200 psi, which means that it could burst at 1600 psi.... This means that a 1400 psi regulator is the most that makes sense from a safety point of view, and then ONLY when protected by a 1.8K burst disc on the downstream side.... Shocked

The next problem is the QB79 tank block.... It mounts with two 4mm screws of questionable quality, installed too close to the end of the main tube.... While it may be OK for CO2 (and even then, I would replace the screws with Grade 5 or better).... There has been at least one well documented failure of the mounting system with HPA.... The regulator failed, and bypassed the full tank pressure (which was 2600 psi at the time) into the main tube and locked the valve in the gun, preventing the pressure from being shot down.... There was either no low side burst disc, it was too high a pressure, or it failed to burst.... The two mounting screws on the tank block then sheared, and the tank/regulator/tank block assembly flew off the end of the main tube and went spinning across the floor, fortunately with no injuries to the surprised individual holding the gun.... A few seconds earlier, it would have hit him instead of the floor.... Before shearing, the two mounting screws bulged the end of the QB79 main tube, to the point that had the screws held and a full 3000 psi been available, the tube would have likely ripped, causing the same catastrophic event.... Shocked

I addressed the mounting weakness of the QB79 tank block in Version 1 by actually bolting the tank block directly to the Discovery valve (with a 7/16" NF Grade 5 bolt drilled to 1/4" ID), sealed with an O-ring.... meaning that the 22XX tube was not even pressurized.... Both the block and valve were secured into the tube so that if a failure occurred, hopefully the tube would contain the parts and the air would simply leak out both ends (no O-rings) and scare the heck out of me.... In Version 2, I got a little braver, and pressurized a very short portion (about 1/2") of the 22XX main tube, mounting the valve exactly as it is in the Disco, while for the tank block, I made up a guage mount, bolted to the tank block as above.... The stem of the guage is a good fit in a hole in the bottom of the main tube, providing extra shear strength.... In both cases, I used 4mm Grade 5 Socket Head Set Screws to mount the tank block in close fitting holes I drilled in the 22XX main tube.... Is it safe?.... I hope so.... Question

.... Others have bolted the QB79 tank block directly into the main tube by drilling another hole between the two O-ring grooves and tapping it for two additional screws.... These screws are driven in flush to the surface of the tank block, and holes drilled in the main tube to fit closely over the heads of the screws (putting the heads in shear, like the mounting screws in a Disco valve).... This modification means that only one O-ring is left to seal the tank block into the main tube, which apparently works fine, as long as everything is perfect.... With any of these modifications, I would strongly encourage the use of a 1.8K burst disc to limit pressures inside the gun to the maximum found with CO2.... Wink

In Version 1, at the beginning, I used a 9 oz. CO2 tank which I pumped by hand to (eventually) 1800 psi.... This eliminates the danger of regulator failure, PROVIDING that you pump by hand.... Note, however, it is an accident looking for a place to happen if you fill from a SCUBA tank, the same as if you fill a 3000 psi tank from 4500 psi.... If the regulator fails (if so equipped), or you don't turn the valve off quick enough, you could easily turn that tank into a bomb.... and they are big enough to kill or maim you.... The only SAFE way to fill an HPA tank from another tank is if the fill tank pressure is equal to or below the safe working pressure of the tank being filled.... Shocked

HPA is a really cool way to power an AirGun.... but PLEASE.... treat it with respect and STAY SAFE.... Twisted Evil

Bob

yourdaddyjoe · Supporting Member

Like Bob Said... 1800 psi Max... when putting pressure in the 22xx tube.

I fill my 2260 Bulk carbine from a 1800 psi regulated tank. NOT THE SCUBA TANK. I get less shots but... I don't have to worry about it blowing up in my face.

I am getting 860 fps with CR Premier HPs and a peak shot at over 900 fps when it hits 1000 psi.

If I am bench shooting.... I can daisy chain the regulated tank between the the SCUBA tank and the AG for all day shooting.

I moded a 2240 valve including drilling and bottom tapping for two side 8-32 screws to lock it in place.

I am very confidant in this set up for a bulk fill.

Edit: I am still waiting on my Diso hammer springs... Needs mo power!

I will say it again. NO MORE THAN 1800 psi in a 22xx tube.