Air Gun Home :: View topic

rsterne · Moderator

A while back I built a custom Tank Block incorporating a Picatinny rail, pressure gauge, and 1.8K burst disc.... I had to wait until after my knee surgery to get back to building a 2260 around it.... The gun weighs just 6 lb. 2 oz. ready to go.... including a 4-12 x 40AO Bushnell scope.... and balances right in the middle of the foregrip:

I used a 13 cu.in. 3000 psi tank regulated to 1600 psi.... I used a Disco valve bored out to increase the volume about 50% (to ~3cc).... The valve is fitted with an extended front end which places the O-rings ahead of the roll-stamped area for safety.... I had a Disco breech and barrel assembly laying around so I used that as well.... including the sleeve where the rear sight mounts.... I added a barrel band to the top of the Tank Block that clamps that sleeve rigidly at the front.... The barrel ahead of that is free floated.... The 2260 Main Tube was shortened so that the barrel band lined up with the front of the barrel sleeve.... It was then drilled for the tank block and valve mounting screws.... I made a custom Muzzle Brake which is only vented on the top to reduce muzzle flip....

Since the Disco breech has the 4-48 mounting screw further back, I had to grind a groove in the top of the hammer to miss it.... I lightened the hammer 25% (to 45 grams) while I was at it.... This limits the top-end perforrmance, but reduces the shot-to-shot variation significantly.... I modified the trigger group by polishing all the contact points and fitted a lighter trigger screw which gives a nice 2-stage feel and lightens the pull to under 2 lbs.... I also fitted one of my Rear Velocity Adjusters, and a Disco hammer spring.... I have a Bi-Pod which will fit on the Picatinny rail on the bottom of the tank block, but it is too high for bench rest shooting.... I made a much lower one using the bottom half of a Weaver scope mount ring and a piece of aluminum bar stock drilled and tapped for two 5/16" bolts fitted with rubber feet.... I can change the length of the bolts to suit my situation.... Here is a detail of the Tank Block & Bipod.... Incidently, the gauge is a very nice 0-2000 psi unit with 100 psi increments.... actually cheaper than a Disco gauge....

I ran the gun through my standard tests, using different pellets at different hammer spring settings to develop the performance graph below.... I added a curve showing the number of shots with JSB 15.9 GR. Exacts shooting from 3000 psi down to 1500 psi....

When I looked at the Energy curves for this gun, I found that even at coil-bind on the hammer, the EunJins were on the down side of the peak.... The highest energy I saw was 37.5 FPE shooting 25.2 gr. JSB Exact Monster pellets at 819 fps.... Unfortunately, as is typical when you strive for maximum power, this setting produced only about 30 shots.... and MAN was it LOUD.... Rolling Eyes

.... From 2 to 6 turns out on the RVA, the highest energy was produced with my favourite pellet, the JSB 18 gr. Exact Heavy.... At 8 turns out, the JSB 15.9 gr. Exacts were the best.... shooting at 620 fps (13.6 FPE).... The gun is VERY quiet and produces about 120 shots to 1500 psi at that setting.... I concentrated on the JSB 18 gr. Exact Heavy for my remaining testing....

One of the things I have found with regulated PCPs is that there is an optimum hammer spring setting to get the flattest shot string.... If you go for maximum power, you get very few shots and the first ones are the fastest, followed by a very slight decline (just a few fps) until you hit the regulator setpoint at which time the velocity plummets.... At the other end of the spectrum, with the power dialed way down.... you get a shot string which climbs very slightly (10-15 fps) until you hit the regulator setpoint, where the velocity the climbs quickly to peak at a lower pressure.... At 6 turns out on the RVA, this gun produced about 75 shots at 705-720 fps (20 FPE with the 18 gr. pellets).... Once the pressure dropped below 1500 psi, however, the velocity started to climb, peaking at nearly 765 fps at only 1000 psi.... This was obviously well below the efficient range for this gun.... At 4 turns out, it produced about 50 shots from 815-825 fps (27 FPE @ 18 gr.) and the velocity only climbed to about 840 fps at 1150 psi.... 2.25 turns out turned out to be the sweet spot for this gun.... I got 45 shots between 894 and 903 fps starting from 3000 psi and continuing flat until 1300 psi.... some 300 psi below the regulator setpoint.... The average was 898 fps (32.2 FPE) with the JSB 18 gr. Exact Heavy pellets.... giving an efficiency of 0.95 FPE/ci.... or 17.3 Barcc/FPE.... Other than a couple of shots (likely damaged pellets) all shots were within 9 fps.... ie just a 1% total variation.... Cool

This gun (and the results from my previous 2260 regulated @ 1900 psi) clearly shows that for any given power level there is an optimum regulator pressure.... With the bored valve, a 0.166" transfer port, and a 0.169" barrel port.... the optimum regulator pressure is about 1000 psi for 20 FPE.... 1200 psi for 26 FPE.... 1400 psi for 30 FPE.... 1600 psi for 33 FPE.... and 2000 psi for 36 FPE.... While it is possible to change the performance within a wide range by simply changing the hammer spring preload.... those are the pressures that produce the best results with my standard modifications in .22 cal.... When you consider that AirGunEric sells 1500 psi regulated tanks that are right in the sweet spot.... it makes the decision pretty easy.... Wink

Bob

sniper · Silver Status Member

nothing less than impressive-as usual.

another badge earned in authority for the professor. Cool

maybe one day i can understand all the math before
my head hurts. Embarassed

packrat · Senior Member

yeah...what Sniper said!!!

Slavia · Moderator

sniper · Silver Status Member

how many pcp threads and slavia still doesn't have one. Shocked

oddtodd · Silver Status Member

I had one and got rid of it asap. THat pumping crap is for the birds and I couldn't get the money together for scba tanks.

Slavia · Moderator

packrat · Senior Member

oddtodd--NOW is a good time to get SCBA set-up...you can get a 66 cu.ft., 4500psi, SCBA tank, with 3f t SSline and gauge set-up delivered to your door for $350.00..Tanks have NEW hydro, the hose/gauge stuff is FIRST CLASS quality!!!!! here's link to his site..I got mine about 2 weeks ago--should have gotten it 2 years ago!!!

http://www.hamcontact.com/airgun/HoseAssembly/

rsterne · Moderator

Got a chance to try it outside today.... JSB Exact Heavy 18 gr. pellets at 898 fps (32.2 FPE).... 5-shot group at 25 yards with the short built-in bipod on the bench and supporting the rear of the stock with my left hand underneath and the butt against my shoulder....

C-T-C is 0.48".... OD can be covered with a dime.... Mr. Green

Bob

Lipoly · Veteran Member

It is amazing that you can get that kind of fpe from 2000psi.

rsterne · Moderator

packrat · Senior Member

Lipoly- please re-read my last post..it was directed to "oddtodd"..just passing on information.

Lipoly · Veteran Member

Mark5043 · Silver Status Member

packrat · Senior Member

Bob.. as is the norm, I'm most appreciative of your summary...i do have a question for you, since you've had the opportunity to use several different tanks and regulator set-ups-have you found any one particular brand name or model that performs better in any way? I only have experience with the 490cc Air Force tank and the 13 cu.in. bottle/regulator from JDS Airman...thanks for any insight..

edited to correct spelling