Air Gun Home :: View topic - New QB 78 on HPA

rsterne · Moderator

I've started working on another QB 78 setup to run on HPA.... The twist on this one is that the tank is reversed and will act as the forestock.... I did this on my Hayabusa project, and the QB 78 main tube was just the right length.... I'm using a 13 CI Ninja 3000 psi tank regulated to about 1600 psi and equipped with a 1.8K burst disc.... The tank block is machined from a block of 1" thick 6061-T6 aluminum that measures 1.37" x 2.63".... It is wide enough for a 3-groove Picatinny rail on the bottom for mounting a bipod and/or sling.... The tank is tucked up against the bottom of the main tube with minimal clearance.... The assembly attaches to the front of the main tube using the stock knurled nut with the hole enlarged to 1/2".... That is the only modification to the gun necessary to attach the tank, other than shortening the stock....

There is a brass fitting with O-ring that fits into the front of the QB main tube and is retained by the knurled nut.... The O-ring is moved to behind the threaded part of the tube for additional strength.... That fitting is attached to the tank block using a steel 1/8" NPT hex pipe nipple, with the knurled nut captured in between and free to rotate.... Below where the nipple screws into the block I machined a standard paintball fitting that the regulator output of the tank screws into.... There is a pin that presses on the pin valve in the regulator to open it.... The air passes around that pin and travels vertically through the block via a 1/8" hole to where the pipe nipple is.... and then from there into the main tube.... The bottom of the 1/8" hole is plugged by a 1/4" long 10-32 low profile SHCS recessed flush with the grooves in the Picatinny rail, and sealed by an O-ring....

Also in the photo above is the highly moddified QB valve.... Basically, once you open up the front of the valve, all it does is provide a place for the spring to sit.... With that in mind, I machined away all the extra material, so the valve is now the entire volume of the main tube, about 50cc including the valve area.... This is a large enough volume of regulated air to support up to 50 FPE shots without losing efficiency.... Since the eventual plan is for this gun to be converted to .25 cal, that is ideal for my purposes....

Bob

rsterne · Moderator

I got the stock shortened today and everything bolted together for the first time.... and there were no leaks.... Here's what it looks like sitting on a bipod....

I made the barrel band later in the day but it's not in this photo.... It fits in the recess in the top front of the tank block.... I machined the extra notch in the breech to hold the bolt back as this gun will be "cock on open" and it makes sure there is lots of room to load the pellet.... I also machined recesses in the breech and valve for a poly transfer port and finished modifying the valve, installing a Delrin seat on the poppet and boring the valve out around it for increased flow.... I also eased the corner at the bottom of the exhaust port and drilled the valve throat out to 0.223".... This gun will use the new hammer I developed for the last QB 78 I built....

You can see the extra cocking notch for the bolt in that photo.... The upper hammer and cocking block are stock.... the lower ones are the new ones.... The sear catch point is moved forward 0.20" to increase the hammer travel, and the back of the hammer is shortened the same amount.... The spring guide is shortened about 1/8" and there is a hole in the bottom of the spring recess in the hammer to miss it and allow the hammer to go all the way back until it touches the cocking block.... That gives maximum possible preload room for the hammer spring....

I will be making a preload adjuster for this gun, but I haven't yet decided if I will just make it off center, or make a new rear block with screws top and bottom to retain it and the adjuster on the centerline.... I'll have a think on that tomorrow....

Bob

rsterne · Moderator

I did a bit more work on this gun today, sorted out the velocity adjuster and installed the barrel band.... Here are the photos....

The tank block installation is now complete.... However, I still have to rebuild the regulator from it's current 850 psi to about 1600.... I decided to install the hammer spring preload adjuster off-center.... The set-screw opposite it presses on a small delrin rod that bears on the side of the adjusting screw to provide a "brake" so that it will hold it's adjustment.... I ground the head of the 10-24 Stainless steel adjusting screw so that it has an indicator point to make full turn adjustments easier to track....

The adjusting screw pushes on a 5/8" long piece of steel round stock that is a sliding fit in the main tube.... If you back the screw out (or remove it), the gun shoots just under 500 fps with 14.3 gr pellets using the 850 psi regulator.... I kept shortening the sliding block until it was the correct length to achieve this.... There is very little preload on the hammer spring set up that way.... From there, I can turn the screw in 11 turns before the bolt can no longer be pulled back to engage the rear cocking notch in the breech.... I can actually go three more turns before the hammer runs up against the cocking block for a total of 14 turns of adjustment.... Should I need the last three turns of preload (highly unlikely) I can machine the cocking notch in the breech ahead more and still have sufficient room to load even the longest pellets....

I did enough shooting to assess the velocity adjuster.... Using 15.9 gr. JSB Exacts, with the adjuster backed out all the way I got about 450 fps.... In 1 turn, it increased to ~600 fps.... At 2 turns in, it was about 710 fps.... 3 turns in, the velocity was 790 fps.... and 4 turns in, it was 805 fps.... Turning it in more after that did nothing to the velocity, it just made the gun louder and used more air.... In other words, I have 7 turns of adjustment left to allow for higher pressures, which based on my previous experience should be more than enough to handle 1600 psi.... I don't want to run more pressure than that, as I want to still run the 1.8K burst disc on the downstream side of the regulator for safety.... It's beginnning to look like this gun breathes pretty well.... as I'm up to 23 FPE running only 850 psi.... That's stock Disco power at half the pressure....

Bob

oddtodd · Silver Status Member

Cool build! I like it Bob. Now if I only had the tools to build it for myself.

rsterne · Moderator

Well today I got to see what this thing would do.... I reset the regulator to 1600 psi, reassembled the gun, grabbed a tin of 18.1 gr. JSB Exact Heavies, and started shooting.... The velocity peaked at about 940 fps, just over 35 FPE, which is pretty decent for that weight pellet on 1600 psi.... I started with the hammer spring tension all the way in, shot 2-3 shots, then backed it out a turn, recording the velocities.... I stopped at 8 turns out.... For the first couple of turns out, the velocity didn't change at all, so I then started shooting strings at 2 turns out.... shot a string and then backed it out another turn, until it started to drop off the plateau at 5 turns out.... Here are the results....

Note how the efficiency climbs as you reduce the hammer strike, even though the velocity hardly changes.... It shows how important this type of testing is in a regulated PCP to not just waste air.... At 2 turns out I only got about 20 shots, at 3 turns out about 30, and at 4 turns out I could get 40 shots from a 3000 psi fill.... That's double the shot count with only a 7 fps loss of velocity.... One more turn out I lost about 30 fps but got 60 shots and the efficiency climbed to 1.32 FPE/CI.... That was a really exciting piece of news because that was 5 fps higher velocity than my previous QB78 was getting at an efficiency of 1.20 FPE/CI when tuned for the same shot count....

To say I'm pleased with the performance of this new gun would be an understatement.... It will be interesting to see what it can do when I get a .25 cal barrel for it.... If I was tuning it for these pellets I would try 4.5 turns out on the spring preload.... I would expect about 50 shots at about 34 FPE tuned that way (~920 fps with 18 gr. JSBs)....

EDITED to add heavier pellets:

I turned the hammer preload in to 3 turns out (to allow for heavier pellets to max out) and got the following results....

18.1 gr. JSB Heavies - 934 fps - 35.1 FPE
21.0 gr. H&N Baracudas - 890 fps - 36.9 FPE
25.3 gr. JSB Monsters - 832 fps - 38.9 FPE
28.4 gr. EunJin Domes - 799 fps - 40.3 FPE
32.1 gr. EunJin Points - 768 fps - 42.1 FPE

Bob

TheAznInvazn · Member

Very nice!

Do you know of places where you can buy barrel blanks?

rsterne · Moderator

Lothar Walther in Georgia....

http://www.lothar-walther.com/457.php

Bob

rsterne · Moderator

I finally got back to this project the last couple of days.... I made a chambering reamer for the .25 cal Lothar Walther barrel and cut the chamber.... The forward pilot portion of the reamer is 0.243" diameter and rides on the rifling lands.... Then there is a 2* tapered section and the chamber portion is 0.255" diameter.... I then machined down the portion that inserts into the breech, milled the flats on it, machined in the transfer port and the O-ring grooves.... I then crowned it and polished it in preparation for bluing....

The photo above shows the bottom of the barrel and breech and the chambering reamer.... Note the much larger (3/16") transfer port.... The next photo shows the top of the barrel and breech, and the bolt, which has been modified to .25 cal....

I made one mod to the breech, I added two 6-32 set screws on the sides at the rear of the barrel.... There are matching flats on the sides of the loading port.... That prevents the back of the barrel from moving, therefore stabilizing the barrel much better in the breech.... The bolt was actually cut off about 3/8" in front of the full diameter portion, drilled and tapped 8-32, and a short stud threaded in.... Then a piece of steel rod was drilled and tapped and threaded onto the stud to make a new front end for the bolt.... You can't even see the joint in the photo.... A new O-ring groove was machined in the appropriate location carrying a 1/4" O-ring stretched onto a few thou oversize diameter so that it seals in the 0.255" ID chambered portion of the barrel....

I still have to make a new 3/16" ID transfer port.... I have found that Teflon rod works extremely well once you determine the correct length to get a bit of "crush".... I hope to do that and assemble the gun for initial testing tomorrow....

Bob

rsterne · Moderator

I finally got the .25 cal version assembled today, and did the initial testing.... I filled the tank, recorded the fill pressure, shot 10 shots, and recorded the ending pressure.... for every hammer spring preload setting from maximum to 7 turns out.... The regulator setpoint on the 13 CI 3000 psi tank is 1600 psi, and the gun is equipped with a 1.8K burst disc for safety.... I was using 25.4 gr. JSB Kings, and here are the results....

You will note that the last 3 turns of preload on the hammer spring do nothing to the velocity, but as usual the air consumption keeps increasing.... That is because the valve is closing too late to do any useful work, so the gun is just wasting air.... Here is the data for the shoulder portion of the curve in table form.... The number of shots are extrapolated from the pressure loss over 10 shots, so may not be exact....

I added a column for the "Bore Ratio", which is the barrel volume divided by the volume of HPA released during the shot.... You can see that once the pellet has travelled half the barrel length (Bore Ratio = 2.0) at the point the valve closes, adding extra air does virtually nothing (except waste air).... The more I experiment with regulated guns, the more I find this to be the case.... If you tune for where the velocity just starts to drop, you can often get the same power and use about half the air that you would running a "dump shot"....

I also tested other pellets with the hammer spring set at maximum, just to get a feel for what the gun can do.... Here is the data....

I'm pretty pleased that I got nearly 60 FPE from a QB 78 running on only 1600 psi.... I will be doing a bit of playing to get the final tune, which will probably be about 4 turns out.... I hope to get about 30 shots at 45 FPE with the JSB Kings.... The only work left to do after that is blue the barrel, stick a scope on it, and sight it in....

Bob

lampy · Veteran Member

Very cool!

rsterne · Moderator

I topped up my SCUBA tank with my ShoeBox this afternoon, and while it was ticking away I blued the barrel and touched up the bluing on the breech and bolt.... Then after dinner I reassembled the gun, set the RVA at 3.5 turns out, filled the tank to 3000 psi and shot a full string.... Here is a graph of the velocity, using 25.4 gr. JSB Kings....

To say I was pleased would be an understatement.... The string ended at 1300 psi, and only the last shot was low, at 907 fps.... I had 32 shots between 915 and 923 fps, an ES of only 8 fps (0.9%).... I was able to get more shots than I was hoping for at that power setting, which worked out to nearly 48 FPE.... That is excellent power from a regulated pressure of just 1600 psi.... Here is a summary of the results....

As you can see, the efficiency was a bit better than I reported previously, at 1.03 FPE/CI.... The gun is fitted with a Bushnell 4-12 x 40 AO scope and weighs 8 lb. 2 oz all up.... Here is a photo of the finished product....

The 13 CI 3000 psi tank acts as the forestock, and the gun balances just in front of the stock securing screw, which I find very nice.... The tank block incorporates a Picatinny rail for mounting a BiPod or other accessories.... I'm very pleased with the outcome of this project and I can't wait to see how accurate the Lothar Walther barrel is.... I am missing the safety lever, so if anyone has a spare, please let me know....

Bob

WestTexasFarmBoy · Member

I just love your stuff Bob -- you are a mad genius!

radar · Silver Status Member

viper2788 · Member

Dude... thats an INSANE shot string! Btw what program are you using for the machined parts? Looks really pro

rsterne · Moderator

program?.... just a lathe with a milling attachment.... all manual....

Bob