New QB 78 on HPA - (Now .25 cal)

[rsterne]

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

[Mudbug]

Sick!!! I cant wait to see what she looks like when it's done .

[rsterne]

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

[rsterne]

I guess the reamer worked OK, huh?.... Thanks Sean....

Bob

[SeanMP]

Holy cow Bob....would you mind printing up a program so that we can keep up. Two days ago we were talking about the leade on a Big Bore and now you already have 50% of a QB conversion done.

How are you machining the threads in the tank block for the paintball tank?

[rsterne]

Painstakingly.... I lay out the position in my milling attachment while drilling the hole for the pipe threads and centerdrill it.... Then I chuck it in a 4-jaw, using the centerdrilled hole and a 60* center in the tailstock for alignment.... I drill it 1/8" to a depth of an inch, and then 1/2" to a depth of 0.7" to the point of the drill.... Then I use a 5/8" end mill to square off the hole to 0.700" deep.... I then use an 11/16" mill and bore in about 1/4" and check the size.... I'm looking for about 0.695" ID, and today I was lucky, so I just bored it in 0.700" and that gave me the shouldered portion for the O-ring to seal on....

I then used a boring bar and enlarged the hole to 0.740" ID to a depth of 0.400".... I then took an internal threading tool, put the point in at 0.400" and opened up a groove to 0.840".... That gave me a spot to stop the threads and also formed a tapered shoulder for the O-ring to guide on when entering the inner portion.... I then set the lathe up for 14 TPI and using five 0.010" cuts I threaded the outer portion until the point of the threading tool was at 0.840" ID.... I stop the feed about 0.010" before the tool touched the shoulder I made in the previous step (ie 0.390" below flush) so as to not damage it....

At this point I then use a 1/2" -14 NPS tap and run it in to clean out the threads, making sure I stop before the tip of the tap touches that all-important shoulder.... I then change to a bottoming tap (I ground one perfectly square on the end) and run it in exactly 5.5 turns, at which point (if I did it all correctly) you can feel it free up as it cleans out the bottom thread.... Again, DON'T touch the shoulder.... With a bit of luck, you don't damage anything (today was a good day)....

I then carefully deburr the outside edge of the hole and the edge of the shoulder, and thread in a tank without O-ring.... Assuming it bottomed (and it did) I double check the transition at the shoulder below the threads where the smooth portion starts for the O-ring and try the tank with O-ring.... If it feels good, and if, on removal, the O-ring looks good and not shredded then it will probably hold pressure.... This one was particulaly nice, so I'm confident it will be fine.... I've done 4-5 of these now, and while they are still nerve-wracking, they are getting better each time.... and even the first one I managed to salvage....

Bob

[rsterne]

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]

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

[Doc Sharptail]

Kind of curious as to the point of diminishing returns on the valve cut away.

I'll ask it here (I may have missed reading it else where)... what increments did you use to work up flow to arrive at those openings? I'm interested in what the process was...

I have an interest in going to .25 cal at a later date, once I've got my 78 accurized...

Regards,

Doc Sharptail

[rsterne]

Here is a previous valve I did.... compared to stock....



My reasoning is that once you remove the piercing pin, internal sleeve, and open up the valve.... you basically no longer have a fixed volume of valve anyway.... The pressure drop in the valve during a shot is then proportional to the volume of the entire tube.... I took this to the extreme in my Hayabusa.... Here is the valve arrangement for that gun....



I have done a lot of work that all leads me to belive that if you are searching for high power, you need to get the air into the valve (ie to the valve seat) quicker than it can get out.... and the higher the pressure you can maintain during the shot cycle, the more performance possible.... That is why I have a design point of 1 cc of air "on deck" for every FPE of energy I want the gun to produce.... My Hayabusa has 110 cc of air between the regulator and the valve seat.... This new QB has 50-55 cc.... A QB 79, even if you installed my completely opened up valve in it only has about 10 cc.... basically just the volume of the opened up valve, there is no tube length.... That doesn't mean you can't get good power out of a QB79.... it just means you can get more from a QB78.... When I used the valve in the photo above in a QB78 I got over 31 FPE.... while the identical setup in a QB79 only produced 26 FPE....

Bob