QB PCP .25



Author Topic: QB PCP .25  (Read 1648 times)

Offline Ribbonstone

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QB PCP .25
« on: February 04, 2012, 03:19:38 PM »

The curse of small air volume is short shot string.  The higher the power, the shorter the string.

Had had this idea on a mental back-burner for some time.  But a chance to do some semi-serious Nutria hunting has come up, so thought it was time to do something about it.

Compasseco once sold PCP QBs and  just the PCP tubes for QBs (they don't now).  Had been using one for a little .22 carbine squirrel gun, down tuned for a long shot count and low energy (15 foot pounds).  Have other PCP/HPAs that fill that need now and that QB PCP has been being neglected.

So I stole the bottom half  of the PCP 22and matched it up to the top half of an HPA  Mountain Air .25 QB.   Have a couple of strikers of different weights, a few springs of different strengths/lengths, and a valve modded for higher flow.  Figure sooner or later would manage a combination that gives standard pellets (25-27gr) 850-900fps with at least a “useful” shot count.

Quick pictorial of the gun's assembly:

Old Compasseco QB PCP.  Removed the top half (breech and .177 barrel).


Added a .25 top half.  Mountain Air bolt and .25 LW barrel (Roy does nice work..a tidy flow though .25 bolt probe):


As simple a valve as I can manage. There is more to be gained here, but will leave that for the 2nd round of testing.


Standard weight striker (cleaned up/polished). Can try different weights as results warrant.


Large transfer port. Some smoothing could gain a bit here as well, but will have to see if it is needed after testing.


Stronger spring adjusted to get the gun running at high pressure.  Will have to play around here with various springs after getting some velocity readings (in day light).


Tried various combinations of strikers/springs/ports, getting as high as 51 foot pounds and as low as 40.  Figure the only real reason for a .25 is power; if I wanted less would pick out one of the other air guns in the safe.  So went with 45-50 foot pounds as my goal.

Today's final outcome.  Great big LDC (it moves a lot of air) which makes for a long barreled rifle.


Shot velocity listed under each 3-shot 20yard target (got to start someplace, and it is easier to work with short yardages before going for longer distances).


So the string really comes out to this:
 920-890fps = 10shots
(for those who like energy, Kodiaks end up at 859fps...which is denctly close to 50 foot pounds).

Now that is a darn short shot string, and my first thought was that I had a lot of work to do.

But maybe not.
(page from notebook)


My best guess to the volume of the air tube is something between 107 and 115cc's.  Ran the math, and it comes out to 107cc. Just a direct comparison to a Discovery tube (which is very-very close to the same OD/ID) shows it to be 20% shorter...and if a Discovery is 135cc's, then the QB tube is 108ccs'  For efficiency calculations, used 112 cc just adding another 5ccs  as an estimate of valve volume.

I welcome anyone who will re-work this to check my figures, but I figure there is not a whole lot to be gained.  IF the gun has a 112cc tube, and the average of those 15 shots is 895fps, then the BAR/CC efficiency comes out to something close to 10.3.

When you start getting efficiency numbers under 12, there is not a whole lot to be gained from fiddling around.  Could certainly level out the velocity a bit and earn a couple more shots...but it seems a whole lot of work for very little gain.

Undecided stuff:

The scope right now is a 6X42 Walther, including that Germain “Enemy at the Gates” fat triplex cross hair.  Will slip on a higher power scope for the upcoming 50yard tests, but I kind of like those fat tri-posts for lower light hunting, so suspect I will go back to it.

Can definitely make a slimmer/trimmer/more efficient LDC. The one shown works fine, but is kind of “bulbous”. Won't take too much to make a nicer looking unit.

Need to take apart the trigger and do a bit of shim/smooth/adjustment work.  Weight is not bad, but can be a bit crisper (and I have a spare trigger assembly, so if I screw this up, and not SOL).










Offline rsterne

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Re: QB PCP .25
« Reply #1 on: February 04, 2012, 04:40:41 PM »
Nicely done.... Sure shows the potential for the humble QBs....

Bob
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal:
.177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine (Grouse Gun), 2260 PCP Rifle (37 FPE), 2560 PCP Rifle (52 FPE), .22 BAM B-26, .22 BAM B-51, Hatsan AT-4410 Long (70 FPE),
"Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, .308 & .357 unregulated; working on the Mk.III, a .410 shotgun, and .458 cal)

Offline Ribbonstone

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Re: QB PCP .25
« Reply #2 on: February 04, 2012, 08:18:43 PM »
Get that reverse-tanker QB tuned up...am interested, both for curiosity reasons and selfish ones.

Selfish because am just missing a barrel to have a complete HPA QB (mostly the bottom half of the gun that donated the .25barrel/breech) and the urge to give high output pressures another try. 

Offline rsterne

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Re: QB PCP .25
« Reply #3 on: February 04, 2012, 11:26:24 PM »
You're running a lot more pressure than I will be.... My regulator setpoint will be below where your shot string ended.... I don't expect to get efficiency numbers as high as yours as a result....

Bob
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal:
.177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine (Grouse Gun), 2260 PCP Rifle (37 FPE), 2560 PCP Rifle (52 FPE), .22 BAM B-26, .22 BAM B-51, Hatsan AT-4410 Long (70 FPE),
"Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, .308 & .357 unregulated; working on the Mk.III, a .410 shotgun, and .458 cal)

Offline Ribbonstone

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Re: QB PCP .25
« Reply #4 on: February 05, 2012, 12:31:08 AM »
Really never have gotten great efficiency numbers with HPA. At least part of it is that regulators do work at each shot...and that energy is supplied by air...but that work is not muzzle energy. Using the standard formula, most of the HPA  13ci guns are seeing something from 18-24 bar/cc.

A .22 (on a Crosman) that can manage 110 shots from 3K-1K, 800psi 13ci bottle, at 670fps/14.3gr. per shot.  Using the standard bar/cc formula (and the on line calculator is so much easier..http://calc.sikes.us/1/index.php ) that manages just under 19bar/cc.

In fact, the spread over the 6 guns running on 13Ci bottles is pretty close...close enough that you can reverse-work the formula...counting on a bar/cc rate of about 20...and come very close to figuring out the shot count for a given energy.

Offline rsterne

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Re: QB PCP .25
« Reply #5 on: February 05, 2012, 01:10:24 AM »
The last QB 78 I worked on had a 13 CI tank regulated at 1500 psi.... I found that the hammer spring was overshimmed, and just correcting that took the efficiency from 0.85 FPE/CI (19 Barcc/FPE) to 1.0 FPE/CI (16.4 Barcc/FPE) and I only lost 3 fps.... That got me thinking, and I worked on balancing the hammer hit like you would in a non-regulated gun for just below the regulator setpoint (ie ~1400 psi).... I only lost another 4 fps average and the efficiency went up to 1.20 FPE/CI (13.7 Barcc/FPE).... That was shooting 896 fps with 18.1 gr. JSBs (32.3 FPE) and I got 60 shots from 3000 psi down to 1200....

I was pretty amazed at that efficiency at those power levels on a regulated gun....

Bob
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal:
.177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine (Grouse Gun), 2260 PCP Rifle (37 FPE), 2560 PCP Rifle (52 FPE), .22 BAM B-26, .22 BAM B-51, Hatsan AT-4410 Long (70 FPE),
"Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, .308 & .357 unregulated; working on the Mk.III, a .410 shotgun, and .458 cal)

Offline djdilliodon

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Re: QB PCP .25
« Reply #6 on: February 05, 2012, 01:19:49 AM »
I can't wait to see what that .25 QB can do at 50 yards plus. Judging from the 20 yard groups it seems you may just have one heck of a tack driver!
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Benjamin Discovery .22 leapers 4-16x50 AO illuminated mildot, second barrel band, TKO 6.5" brake, custom power adjuster.

Stoeger X10 .177 Leapers fixed 4x AO mildot.

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Offline Ribbonstone

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Re: QB PCP .25
« Reply #7 on: February 05, 2012, 01:35:57 AM »
Weather will decide if I take a quick crack at 50 yards tomorrow or not.
I don't like 3 shot groups, used them in the above tests looking for POI changes through the sweet spot, so don't count on 3 shot groups as a real test.  Really less POI change than I expected; evidently the high BC pellets do well enough that small vel. variations don.t show up as fast as when using low BC pellets.




Offline djdilliodon

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Re: QB PCP .25
« Reply #8 on: February 05, 2012, 02:57:55 AM »
Weather will decide if I take a quick crack at 50 yards tomorrow or not.
I don't like 3 shot groups, used them in the above tests looking for POI changes through the sweet spot, so don't count on 3 shot groups as a real test.  Really less POI change than I expected; evidently the high BC pellets do well enough that small vel. variations don.t show up as fast as when using low BC pellets.

I agree, 3 shot groups won't tell the whole story as far as accuracy goes. I like to do at least 5 shots per group. I can relate to not wanting to shoot depending on weather conditions. I took my disco for the first time since I got it to try out at 50 yards. I was shocked that with jsb 15.9 I was able to put 5 shots within .4-.7" groups. This wind was blowing lightly to the right but IMO in ideal conditions I'm confident it would average .5". I tried the disco later on in the day at 100 yards and the difference was night and day. The wind picked up and was blowing left to right. I shot the first group and it measured 6" ctc and the shooting ended lol. The first shot I took putting the scope at 12x and using the last mildot put the pellet dead center in the bull with no wind. By the second shot and there after those jsb's would get blown around like nothing!
  • Long island, NY
Benjamin Discovery .22 leapers 4-16x50 AO illuminated mildot, second barrel band, TKO 6.5" brake, custom power adjuster.

Stoeger X10 .177 Leapers fixed 4x AO mildot.

More to come!

Offline Ribbonstone

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Re: QB PCP .25
« Reply #9 on: February 05, 2012, 11:51:24 AM »
50 yards.

Readjusted the trigger a little bit to get it crisper.  Like enough trigger weight to feel when my hands are cold, so didn't lighten it (about 1.5 pounds).

Left the 6X in place.  Just not a lot of time to play swap-the-scope and that 6X suits the way the gun will be used.

Used the center of the sweet spot, which is only 10 shots long, so only two targets.  Was running out of time, and the wind was getting worse anyway. Plinked a few tin cans for fun and drove back.

Basics:
25.4gr. JSB @ 910fps
Scope 1.7” above bore line
Zeroed @ 20yards. That is NOT the sight setting I'll have for hunting.



Thats really not bad, but the twitchy wind was hosing me. If it would just settle down into a repeating pattern, could deal with it better.... but it twitched and shifted in little swirls in no real pattern. A lot more horizontal spread than vertical.

Just one test, but it has me wondering the approximate real-world BC of those pellets in this barrel.  A BC in the .035-.037 range would fit into the drop figures, but that seems kind of high.



Offline rsterne

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Re: QB PCP .25
« Reply #10 on: February 05, 2012, 02:08:49 PM »
I haven't measured the BC of the .25 cal JSB Kings.... but their smaller brother, the 18.1 gr. .22 cal Heavies run up to 0.042 depending on velocity.... so your 0.035-0.037 estimate is likely right on.... The .25 cal Kings have a slightly lower SD than the .22 cal Heavies.... but only a whisker.... Man, that thing is a tack driver....  :o

Bob
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal:
.177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine (Grouse Gun), 2260 PCP Rifle (37 FPE), 2560 PCP Rifle (52 FPE), .22 BAM B-26, .22 BAM B-51, Hatsan AT-4410 Long (70 FPE),
"Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, .308 & .357 unregulated; working on the Mk.III, a .410 shotgun, and .458 cal)

Offline Ribbonstone

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Re: QB PCP .25
« Reply #11 on: February 05, 2012, 03:19:48 PM »
Not that great a shot...but a good barrel and a heavy gun make up for at least some inattention.

This rifle won't get used much...not much need for a 46-50footpound back yard rifle, and that is what 97% of my shooting is.   Call it a 3% rifle, and only about 3% of my shooting needs that kind of zap. Know I could detune it to about 40fpe and get a lot more shots, bjut other than punching paper, those extra shots don't really fill a need.

So of 20 air powered rifles, only four or really powerful...and they get used the LEAST.  A near 50 foot pound .25, a 40 foot pound .22, a 32 foot pound 5mm, and a 20 foot pound .177.  The rest of them are set for either moderate (10-15 foot pounds) or lite (4-8 foot pounds) and get used a lot more often.
« Last Edit: February 05, 2012, 03:21:43 PM by Ribbonstone »

Offline Ribbonstone

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Re: QB PCP .25
« Reply #12 on: February 06, 2012, 11:09:31 PM »


Being addicted to medium speed PCPs, the low shot count  nibbled at my mind.

Down tuned it a bit by way of a new smaller transfer port.  No other changes, and it runs in the same sweet spot for pressure. Normally, a restricted transfer port lowers velcity and increases shot count.  How much it lowers the velocity depends on how much “wasted” gas was being spit out at the original setting.

Yesterday, the 15 shots from 2600-1700psi  earned 690 foot pounds, but only 460 foot pounds fell inside the 30fps velocity-varaition limit.  Today's strangled off transfer port only produced  638 foot pound from the same 2600-1700psi.  BUT all 638 foot pounds fell inside the 30fps velocity limit.

Lost about 10% energy (from 46 foot pounds to 41.5 with 25.4gr. Pellets and from near 50 to 45 foot pounds with Kodiaks) and gained 50% in shot count. Went from 10 shots inside of 30fps to 15shots inside of 30fps (and 11 of those inside of 20fps).

So consider this...am getting less total energy for the air used, but much more energy  that can be put to predictable use. 

Offline rsterne

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Re: QB PCP .25
« Reply #13 on: February 06, 2012, 11:18:38 PM »
Love it!!!   You shaved the top off the curve and flattened it out by strangling the port....

Bob
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal:
.177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine (Grouse Gun), 2260 PCP Rifle (37 FPE), 2560 PCP Rifle (52 FPE), .22 BAM B-26, .22 BAM B-51, Hatsan AT-4410 Long (70 FPE),
"Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, .308 & .357 unregulated; working on the Mk.III, a .410 shotgun, and .458 cal)

GTA - What's been your success with air gun target practice?