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

[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