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rsterne · Moderator

As most of you are aware, I have been working on various versions of my 22XX PCP rifles over the last few months.... My last project was a 2260 that put out 50 FPE.... This time I dialed it down a bit and built a rifle for Hunter Field Target (HFT).... The goal was to achieve 20 FPE for a minimum of 70 shots so that an entire course of fire could be shot without refilling.... I used a 13 cu.in. 3000 psi tank regulated to 1500 psi from AirGunEric at www.scopesandammo.com for the power source.... The tube is cut down from a 2260.... and it uses a stock Discovery valve back end with my extended front end so that the roll-stamped portion of the 2260 tube is not presurized for safety.... The valve volume and internals remains stock, however, as do the transfer port and barrel port.... The tank is mounted with a QB79 tank block, modified with a 3/16" shear pin plus an additional 8-32 screw with the head in shear for safety.... I used the stock components from a 2289, but I raised the comb and made a drop mount for the forearm to customize the fit.... No parts are adjustable to comply with the HFT rules.... I am very pleased with the ergonomics.... the rifle fits well standing, kneeling, sitting on a low stool, or prone.... I fitted a breech band similar to the ones I used on my uber-pumpers, added a set-screw to the barrel, and free-floated it.... I fitted one of my RVAs so that I could adjust the hammer spring tension to set the FPE as desired.... The rifle is topped off with a Bushnell 5-15 x 40 AO Legend scope.... The total weight is only 6 lb. 7 oz. ready to shoot.... and it balances nicely, 2" ahead of the trigger guard.... Cool

Initial tests showed that the Disco hammer spring was too strong.... I could back it out 4 turns with virtually no loss of power.... All it did in the last 4 turns was gulp more air.... and I could only adjust over a range of 16-22 FPE.... I swapped it out for a hammer spring from a 2289 and customized the adjusting screw for maximum travel.... I was delighted to find that I only lost a small amount on the top end, but the rifle could still exceed 21 FPE so that didn't matter.... At the bottom end, however, I can now dial it down to just over 500 fps (about 5 FPE).... with hundreds of shots.... Here are the power curves:

The primary setup will be at about 2 turns out, using pellets over 10 grains at just over 900 fps to develop just under 20 FPE to comply with the FT rules.... By backing the adjuster out about another 4.5 turns, I can dial the power down to just under 12 FPE.... This setup would be used to comply with the International rules.... likely shooting JSB Exacts at ~800 fps.... I didn't shoot a complete string at that level, but I would guess about 130-150 shots.... I can't believe how quiet the gun is at 12 FPE.... and it just gets quieter and more shots as you dial the adjuster out from there.... I'm guessing it will shoot over 300 shots per fill at ~600 fps with 7.9 gr. pellets for practice.... Shocked

After sighting in the scope, I pumped the tank up to 3000 psi, set up targets and the Chrony, and shot the gun until the velocity started to drop off.... I used two different pellets just for fun, alternating 5 shot groups with JSB Exact Heavy 10.3 gr. and 5 shot groups with Crosman Ultra-Magnum 10.6 gr..... Each group was shot at a separate target, and the high, low, and average velocity was recorded.... I shot 80 shots before any significant drop-off, and 85 shots were usable, by which time the tank was down to 1300 psi.... I stopped shooting at 90 shots and went to check the targets.... The gun prefers the JSBs at the moment, but I will be getting some Crosman CPH Boxed soon, so we'll see how those work.... Every 5-shot group with the JSBs was just a ragged hole at my 20' indoor range.... and all 45 shots were under 1/2" OD when superimposed.... The target sitting on the chart was 5 shots with the JSBs.... the small squares are 0.1".... Cool

.... The chart below shows the results:

This hammer spring setting turned out to be almost perfect from a maximum power point of view.... With the JSBs, velocity ranged from a low of 905 fps to a high of 928 fps (18.7 - 19.7 FPE) a range of only 2.5 % (23 fps).... The average was 918 fps (19.3 FPE).... The Crosmans (using the 85 shot cutoff point) ranged from a low of 897 fps to a high of 918 fps (18.9 - 19.8 FPE) a range of 2.3% (21 fps).... The average was 909 fps (19.4 FPE).... Every shot was under the 20 FPE limit.... Using ChairGun to calculate the vertical dispersion from these velocity variations gives the same results for both pellets.... just 0.133" at 55 yards.... less than one pellet diameter.... I know for a FACT that I can't shoot even close to that well.... so I've got 85 usable shots per fill.... Incidently, it only takes about 3 pumps for every 2 shots fired to refill the tank.... Cool

To say I'm pleased with these results would be an understatement.... I chose to start with a mixed bag of components.... but in fact, this would be a VERY easy mod to a Discovery.... All you would need to do is shorten the stock, cut off the tube and (safely) mount a QB79 tank block and an HPA tank.... You would probably need to fit an RVA and play with the hammer spring a bit to fine tune it if you were going to use it for FT.... but if you weren't, a 1500-1600 psi regulated tank is right in the sweet spot for the Disco pressure-wise.... and you'll get a LOT more shots with less variation.... Shocked

As always, if you are playing around with HPA.... BE CAREFUL !!!

Bob

broommaster2000 · Moderator

Hm, can't say it improved on looks... Neutral

sniper · Silver Status Member

come on broom. you know you're just jealous.

i sure am. Mr. Green

rsterne · Moderator

In order to comply with the directive of the Canadian field Target Association Board of Governors, the foregrip of my rifle has been changed.... The post below deals with that change, and since this forum was not involved, it is best to let sleeping dogs lie:

First of all, I would like to apologize for the following statement I made:

rsterne · Moderator

Here is how the gun performs set up for International Field Target.... well almost.... Rolling Eyes

This is a preliminary setup and is a whisker over the 12 FPE maximum.... so please don't jump on me, all it requires is a 1/4 turn more out on the RVA.... What I was interested in was just doing some testing.... How fussy is it for pellets.... how many shots per fill.... velocity variation over the string.... and how the velocity was affected when the pressure went below the regulator setpoint (1500 psi).... Here is the graph, showing the average velocity during each 10 shot string:

The initial shots were right at 800 fps, but as the regulator output pressure drifted downwards (as the tank pressure fell), the velocity drifted upwards.... The total change was just 20 fps between 3000 psi and the setpoint of 1500 psi.... and the gun delivered about 180 shots to that point.... Once the tank pressure dropped below the regulator setpoint, then the velocity began to rise.... It peaked at 842 fps at 1200 psi and then started dropping again.... The velocity returned to 800 psi at around shot # 246.... Shocked

If you were shooting a match, you could fill to about 2500 psi and have 100 shots available before the pressure dropped below 1700 psi, well above the regulator setpoint.... The average velocity over that 100 shots only changes by about 4 fps.... although the peak to peak swings are about 10-12 fps within each 10 shot group.... If the velocity was adjusted so that the highest velocity was 800 fps with Exacts the energy would be between 11.5-11.9 FPE over those hundred shots.... That works out to about 1.5 FPE/c.i. which is very efficient.... Cool

There is an alternative way to set up this rifle if I wanted to shoot in the Int'l Division, of course.... but it would involved a regulator change and that takes it beyond the realm of a simple adjustment.... I could purchase a regulator with a setpoint of about 1000 psi (at which the velocity is currently about 835 fps and falling).... and then dial down the hammer strike so that the velocity stayed below 800 fps.... Then the velocity would be nearly a straight line until the setpoint, and then it should fall off instead of rise.... This would have the benefit of giving well over 200 shots (probably nearer 250) with minimal velocity variation.... I'm just curious about what is allowed and what isn't.... It would be pretty cool if I could change the gun from 20 FPE to 12 FPE with just the RVA (legally).... Wink

While I was shooting all those shots over the Chrony, I also tested seven different JSB pellets.... I shot one pellet as a standard to draw the velocity curve for 5 shots and then shot a different pellet for the next 5 shots.... that way, I could get the information for the velocity curve and at the same time test how various pellets grouped over the entire shot string.... The results are shown below:

The top row of targets were shot first, then the bottom row, then the top again, lastly the bottom row.... From L to R, the pellets are (all JSB):

7.9 gr. Express 4.52 mm
8.4 gr. Exact 4.50 mm (old batch)
8.4 gr. Exact 4.52 mm (old batch)
8.4 gr. Exact 4.50 mm (new batch)
8.4 gr. Exact 4.51 mm
8.4 gr. Exact 4.52 mm (new batch)
8.4 gr. Exact 4.53 mm

While all of the 10-shot (actually two, 5-shot) groups can easily be covered with a dime, there is a clear preference for the 4.52mm Exacts, with both batches being better than all other pellets tested.... Although the targets were only shot at 20' (with a front rest).... they are consistent enough to show pellet preference.... The smallest group was just over 1/4" OD (~0.1" C-T-C).... Cool

All in all, I'm very pleased with the results of this testing.... I was mostly just curious what would happen if I set the gun up for 12 FPE.... The gain in efficiency is huge, doubling the usuable shot count compared to the 20 FPE setup.... Twisted Evil

Bob

Teryx · Veteran Member

Another very interesting demonstration of what is possible with these guns!
Having been involved in some very similar entanglements as the one you described over many years, I've come to the opinion that the rules written by governing bodies have far less to do with providing a clear playing field than with maintaining control and power. It doesn't matter whether you are talking race cars, rifles, or marbles, you find the same thing everywhere you go. For the people at the top it is far more about winning than it is about competing, and a win by disqualification suits them just as well as a win by score. That's why I haven't competed at anything for years. Just because they have the opportunity to clarify things doesn't mean they will. Vague wording is the ace up their sleeve and they know it. Anytime you win, and especially if you upset the usual order, you can expect a technical challenge. The days of accepting defeat as a gentleman are gone. For people who believe that competition should bring out the best in both man and machine, the barriers can be an exercise in frustration. It's not for me anymore. I'm free to just be better without some short sighted committee trying to hammer me flush with the surface.
Teryx

rsterne · Moderator

I finally had the opportunity today to get outside and do some testing at 25 yards.... I started out with the gun adjusted to be just below the 20 FPE limit and tested both JSB 10.3 gr. Exact Heavy and Crosman 10.5 gr. Premier Heavy (boxed) pellets.... Right off the bat, all groups could easily be covered with a dime.... I sighted the gun in with the JSBs as they seemed to have slightly smaller groups.... They I started backing off the velocity adjuster 1/6th of a turn at a time (which loses about 5 fps each time).... As I reduced the velocity, the groups initally tightened up and then started to increase again.... I settled on about 2.5 turns out from coil bind on the RVA which when checked was an average velocity of about 890 fps (18.1 FPE).... Although the group size is only slightly larger at ~19 FPE I thought it would be pretty neat to shown just what this platform can do.... The group below was shot from the bench with a front rest only, and my left hand supporting the butt which was against my shoulder:

This group is 5 shots at 25 yards, using JSB 10.3 gr. Exact Heavy pellets.... It measures 0.32" OD which works out to 0.15" C-T-C.... 8) .... It's pretty amazing what can be accomplished with what started out as a Crosman 22XX.... Mr. Green

Bob

Lipoly · Veteran Member

Holy crap thats good grouping!

broommaster2000 · Moderator

That's not a group any more, that's a hole, and that means you're doing it right! Mr. Green

rsterne · Moderator

Today's experiment is various muzzle weights to determine if I can improve accuracy by playing with barrel harmonics.... I got this idea a while back when I found out that I could reduce the group size be finding the optimum velocity.... It turned out that the gun was much more accurate at 18 FPE than at 20 FPE with no muzzle weight.... I had a couple of muzzle brakes which were basically small muzzle protectors for my .22 PCP Carbine and 2260 PCP Rifle.... so I made several more with only about 5-6 grams difference in weight between each one.... The lightest is about 11 grams and the heaviest is about 39 grams.... so basically 6 weights with a total difference of 1 oz.... They are made of aluminum, and vary from 5/8" to 7/8" in OD.... and have a consistent outlet that doesn't interfere with the gasses exiting the muzzle.... It is deeply countersunk to within 0.020" of the muzzle, where it is 11/32" ID:

I did my first trials inside at 20' and was pleased to discover that there was, indeed, a small and consistent difference between the different weights.... The third heaviest weight.... which happens to weigh 1 oz. (28 grams).... consistently produced the smallest groups.... When I moved outside to 25 yards.... I found the same pattern.... With no muzzle weight, the gun was shooting just over 1/2" OD groups.... basically the same as it did the last time I tried it at 20 FPE.... This time, however, instead of dialing down the velocity, I gradually added weight to the muzzle.... The results are tabulated below:

No weight: 0.58" OD
~11 gr: 0.54" OD
~18 gr: 0.50" OD
~23 gr: 0.44" OD
~28 gr: 0.32" OD
~33 gr: 0.48" OD
~39 gr: 0.56" OD

As you can see, the barrel harmonics are such that there is an optimum barrel weight where the gun produces the smallest groups.... This is exactly the same effect as I saw earlier with no muzzle weight but changing the velocity.... The smallest group size is identical at 0.15" C-T-C .... but this time I am achieving it at 930 fps instead of 890 fps (JSB Exact Heavy 10.3 gr.).... Mr. Green

Bob

fritz · Silver Status Member

I would mix the 28g weight with the lower velocity.

.15" CTC is Olympic level, no?

rsterne · Moderator

I tried lowering the velocity with the 1 oz. weight in place.... the groups opened up.... I'm guessing that each pellet / velocity combination has it's own ideal muzzle weight.... Rolling Eyes

Bob

fritz · Silver Status Member

It's still a very SWEET spot. Twisted Evil

rsterne · Moderator

I tore apart the gun over the past couple of days and changed the configuration.... Instead of a shortened 2260 tube and a QB79 tank block.... the new version uses a full length 2260 tube, Disco fill fitting, some pipe fittings, a tank fill valve and a hose clamp to mount the tank underneath.... This has several advantages:

1. It increases the volume between the regulator and the valve.... in this case over 4 times the volume, 70 cc instead of 16 cc....
2. The increase in volume increases the average pressure during the shot cycle.... according to my spreadsheet, from 1370 psi to 1470 psi (the regulator is set at 1500 psi)....
3. The increase in average pressure means the gun can develop more FPE if desired.... On a FT gun, it means I can turn down the RVA....
4. Reducing the hammer spring preload with the RVA lightens the trigger pull slightly....
5. The higher average pressure and lighter hammer hit increases the efficiency.... it has gone from 1.08 FPE/ci to 1.20 FPE/ci.... (now 13.7 Barcc/FPE)
6. The increase in average pressure allows me to shoot the tank down further.... I now have no significant velocity loss down to 1100 psi instead of 1300 psi....
7. The increase in total volume, plus the greater efficiency, plus a lower refill pressure.... increases the shot count.... It has gone from 85 shots to 110 shots....
8. The gun no longer has a forestock so there is no longer any argument about a knee riser....
9. The new tank placement improves the balance of the gun....

All testing was done with 10.3 gr. JSB Exact Heavy pellets.... The velocity starts at about 921 fps average for the first 30 shots.... increases to 923 average for the next 30.... increases to 925 average for the next 30.... and then drops to 918 fps average over the next 20 shots.... Over the 110 shots the average is 922 fps (19.4 FPE) with an SD of 5.0.... and the extreme spread was 910 to 934 fps.... The velocity peaked at 1300 psi and I tried many shots in that pressure range and all of them stayed under 20 FPE (just).... If I use 1400 psi for a cutoff, the velocity stays between 915 and 929 fps.... within 7 fps of the average.... That still gives me ~95 shots, plenty for a match.... The vertical dispersion over that velocity range is only 0.08" at 55 yards (less than 1/2 the diameter of a pellet)....

Since the barrel is free floated and the velocity is still the same.... there should be no change in the accuracy.... I still have to get out and shoot some targets at 50 yards and will post them once that is done.... Wink

Bob

yourdaddyjoe · Supporting Member

Fun Gus.... Too damn windy to go out and shoot Air Guns... and I am going stir crazy....