Velocity vs barrel length in QB

[Austringer]

Considering different configurations for the new QB 79 I have on order (now for almost 2 months). I'm thinking of shortening the barrel length to make the gun a little more manageable length in the case I choose to build a shroud. With the QB79, I will likely be running 1500 psi on the tank regulator to compensate for the small plenum volume. I assume that will make a difference on the velocity change given a specific barrel length reduction. My desired velocity is mid 800's with 18.1 JSB's. For this build, shot count is unimportant as long as it is more than 40 shots per fill (I'm getting about 70 as similar performance in my QB78.

So, the question is, how much barrel do you think I can chop off a QB79 and retain my desired performance? 2"? 4"? Should I expect issues other than velocity reduction?

Thanks Guy's
Troy

[rsterne]

You will lose about 30-40 fps by shortening the barrel 4".... so it depends on how close to the maximum velocity possible at 1500psi you are tuned now.... If your gun can shoot 900 and you have it tuned for 800, then you will probably be OK.... If it's maxed out to get 800 with your existing barrel, then you will lose more velocity.... The harder you are pushing the gun, the more shortening the barrel will cost you in power.... In addition, the small plenum volume in a QB79 will make it harder to hit your power goal than with the much larger QB78 tube....

Bob

[Austringer]

Thank you Bob. As I don't have the 79 yet, I don't know my max velocity, but based on experience posted by you and other folks here on GTA, I'm hoping the potential velocity will be high enough I can shorten the barrel and meet my goal. It's possible the 79 will balance well enough the extra length won't be an issue, but I really like the idea of a shorter gun. Heck, I might even build a bullpup out of it, but I really like what you did with your custom stocked QB79.

Thanks again
Troy

[wll2506]

Thank you Bob. As I don't have the 79 yet, I don't know my max velocity, but based on experience posted by you and other folks here on GTA, I'm hoping the potential velocity will be high enough I can shorten the barrel and meet my goal. It's possible the 79 will balance well enough the extra length won't be an issue, but I really like the idea of a shorter gun. Heck, I might even build a bullpup out of it, but I really like what you did with your custom stocked QB79.

Thanks again
Troy

I know from where you come on your idea.

I  have two QB79's on order for about 2 months and I'm very seriously considering cancelling them ... not because I'm tired of waiting, nope, it's the fact that now that I can modify my QB78's and since they have more plenum, hence more velocity per charge, (more air volume, all things being equal)

The fact that I have a few 22ci tanks, Ninja SHP regulators and now know how to boost the outgoing to what I want (1250-1300psi),  I'm happy

I went through all my experiments and now know exactly what fittings to order from McMaster-Carr, and have the Teflon tape to make sure nothing leaks and now that I know what to look for and how to do the modifications makes this a relatively 3-4 hour process.  . . and I end up with a gun that has more velocity that a '79 if all other things are the same.

I also like the tank against the stock for safety reasons as it is less likely to get broken off if you should have a serious fall.

The only thing about the '78 modification is I do think a couple of screws into the steel plug would add a good bit of security. As soon as my machinist has a free moment, I will have a custom one made where I do not have to take off the threads of the tube. It will be a plug about an inch, maybe less the length of the standard unit that is in the gun now.

i have a couple of '79 already in 22 and two AR2079A's in 177 for target ... I think I have enough '79's ------- after writing this, i think I will call AGD to cancel my back-order.

Big Five this weekend is having a sale on Beeman Deluxe QB78's in 22 cal for $99.99 and if you look on line there is an extra 10% discount coupon ... for $89.99 for a '78 deluxe, that is hard to beat, especially since we use it as a basis for our modifications !

wll

[nervoustrigger]

Hi Troy, I'm subscribing to follow your progress.  Suffice it to say the air flow channels need to be opened up pretty aggressively to get 30fpe out of a QB79.  Definitely ups the difficulty with a shorter barrel.  Hollowing out the tank block plug helps gain some plenum, and try to get rid of as much of the front half of the valve as you can.

Good luck with your build!

[Austringer]

Jason
I realize the 78 has far more plenum volume and I have considered modifying another 78 to suit my purpose, however ordering a tank block (with shipping) from the supplier cost's half as much as a new 79 from AD! Besides, I'm also interested in reduced weight of the 79 and smaller barrel which I hope might allow room for a shroud with a slight breach spacer. Mostly, I just want a 79 so I can personally compare the attributes of both cheap airguns because...I'm cheap.

Did I read correctly, you are concerned with the tank hanging out in front of a 79? That might mess up my plans a little as while my tank mounted under my forearm is quite solid (will be more so eventually), I don't like the feel of it when hunting. It's too big. It's fine for the bench though.

I learned only recently Big 5 carrys QB78's. That's pretty cool cause I can buy one with cash and my wife will never know! Not that were broke and have no money for food, it's just that she needn't know I'm potentially acquiring another time consuming hobby! Bothers me QB's have been on order for so long at AD, but I will wait.

Another consideration I've had was to fab a new, smaller air tube to replace the clunky bottle. It probably wouldn't make the gun much lighter, but it might make it less cumbersome in the case of a rear mounted bottle on a 78 or leave more room for a LDC on a forward mounted 79. All kinds of ideas and too many other projects in the was.

A dangerous question, don't strike me down; does anyone use welded tubes? High pressure tubes are used in hydraulic service all the time...oops..gotta go...wife is anxious to get going. Continue this later.
Troy

[nervoustrigger]

Yeah, I'm strapped for time at the moment as well.  I'll get back with you this evening.

[wll2506]

Jason
I realize the 78 has far more plenum volume and I have considered modifying another 78 to suit my purpose, however ordering a tank block (with shipping) from the supplier cost's half as much as a new 79 from AD! Besides, I'm also interested in reduced weight of the 79 and smaller barrel which I hope might allow room for a shroud with a slight breach spacer. Mostly, I just want a 79 so I can personally compare the attributes of both cheap airguns because...I'm cheap.

Did I read correctly, you are concerned with the tank hanging out in front of a 79? That might mess up my plans a little as while my tank mounted under my forearm is quite solid (will be more so eventually), I don't like the feel of it when hunting. It's too big. It's fine for the bench though.

I learned only recently Big 5 carrys QB78's. That's pretty cool cause I can buy one with cash and my wife will never know! Not that were broke and have no money for food, it's just that she needn't know I'm potentially acquiring another time consuming hobby! Bothers me QB's have been on order for so long at AD, but I will wait.

Another consideration I've had was to fab a new, smaller air tube to replace the clunky bottle. It probably wouldn't make the gun much lighter, but it might make it less cumbersome in the case of a rear mounted bottle on a 78 or leave more room for a LDC on a forward mounted 79. All kinds of ideas and too many other projects in the was.

A dangerous question, don't strike me down; does anyone use welded tubes? High pressure tubes are used in hydraulic service all the time...oops..gotta go...wife is anxious to get going. Continue this later.
Troy

Troy, what size tank are you using for your backwards tank adaption ?

As I'm writing this I'm trying to cancel my back order from AGD, as I just went to Big Five and they gave me rain check that included the 10% discount. and the $99.99 price for the gun --- cost $89.99 plus tax of course.

 I will order 3 more 22ci tanks tomorrow, as they are in kind of in short supply.

As I just finished writing this thread, AGD just issued a refund and the back order has been canceled ... perfect, I'm taking the Woods Gun out to see if we can get another starling at 65+ yards ;-  )

EDIT;

No more starlings at 65yds, but man alive does this thing shoot, those 10.5 gr Kodiaks are there right now and with very little noise.

wll

[Austringer]

William. Sorry I got u confused with Jason. My tank is a 13ci ninja. I really am not sure if I care for the rear under mount. I'm thinking of making a smaller tank.

[Ribbonstone]

Last time I did a barrel-cut experiment, was surprised.
(Not a QB, but it runs on lower pressure air, so at least some use to your thoughts.)

Same gun, same settings, same barrel, just cut back 10 inches of barrel:




So what was 10” of .22 barrel worth in speed?



On a guess, I had already gotten that little rifle to produce fast little burps of air, so there wasn’t a lot of “extra” air being produced. 

So I haven’t a clue what velocity change you’d see with just a 4” reduction.



Know I lost some speed with a QB79 when going to 16" of barrel, but you can normally retune the rifle to get it back (YEAH...likely that retune would have been faster in the long barrel).

Will look for the old date before the cut....but it's been years.

[rsterne]

While not impossible to get a .22 cal QB79 up to 30 FPE, doing that while staying low enough on pressure to still use a 1.8K burst disc without blowing it won't be too easy.... and that's with a stock barrel.... Since you will be pushing the gun to achieve that, shortening the barrel will have a more dramatic effect....

Bob

[Ribbonstone]

For 30 foot pounds...would want the long barrel.  Suspect that the harder you push 'em, the more benefit to a longer barrel.

[Austringer]

Bob
As you recommend, I have no intention of pushing the pressure on the regulator. Not only do I NOT want to test the integrity of the airtube and tank attachment, but I don’t want to blow burst disks. I don’t have any extras and I don’t care to search for any to purchase. The ninja tank on my 78 is regulated at 1375 psi. If I ever get a 79, I’ll probably adjust it to 1450 or so. I know other folks have adjusted theirs higher, but I don't want to risk anything to ruin my hunt, my shooting or my day. Based on what I’m reading and what I have read previously posted by you and Jason, It is likely I will need the extra plenum volume of the QB78 if I want to shorten my barrel by 4 inches and maintain 30 fpe. As Ribbonstone suggests, a quick high presssure shot of air would probably work better in a shorter barrel than a slower lower pressure (higher volume) one would.  However, I don't have a tank block, or a QB79, so I can’t experiment. I just spent 5 hours driving and pondering all my options. As soon as I got home, I cranked out a shorter experimental LDC, but I start work tomorrow so not al lot will be happening with this project for a while.

Any recommendation’s for smaller diameter, lighter weight options to replace the ninja 13 ci tank (keep the ninja regulator)?

Troy

[nervoustrigger]

Hi Troy, yeah as Bob was saying it's not easy to hit 30fpe with JSB heavies in a model 79 while staying below the rupture point of a 1.8k burst disk.  But give it a go and see how you fare.   If you don't quite get there, it can still be a very successful hunting rig.  Case in point, when my B51 was tuned to 25fpe with JSB heavies, a lot of raccoons went down from well placed shots to the brain from 45 - 50 yards at night.

While I'm here, I wanted to show you a simple method for making a high-flow valve.  Rather than milling or cutting away the front half of the valve or making a pedestal to support the valve spring, simply move the spring to the other side.  All you need is a 3/32" E-clip and a spring from an ink pen.  Hopefully the pictures illustrate it well enough but let me know if you have questions.





This weekend was my first test run for this type of valve in a QB78 being put together for Ron.  It didn't work quite as well as I hoped but with heavy pellets it managed 40.7fpe on 1600psi:


And right at 30fpe with CPHP weighing half as much:


So overall I consider it a success and the fact it's so easy is the icing on the cake.

-------------------------------------------------------------------------------------
edit - Turns out the transfer port was slightly misaligned.  After addressing it, the Eun Jins pulled up to almost 45fpe:

[rsterne]

There are no HPA tanks smaller than 2" OD that I am aware of.... you would have to make your own.... and that is a job only for those experts who really know how to do that safely....

You should be able to achieve 30 FPE at 1400ish psi with a QB78 tube (about 50 cc).... I managed 30.5 FPE (18.1 gr. JSBs) with almost 40 shots with a QB78 tube that was shortened 3" and running on 1500 psi.... but the efficiency was only 0.85 FPE/CI, which means I was pushing the gun pretty hard to get that.... Shortening the barrel on my gun would have cost power for sure....

Bob

[Austringer]

Can anyone tell me the difference in length between a QB78 and QB79 tube? Does a volume of about 6.5 cc’s per inch sound about right in a QB tube? All things considered, I guess I’m probably more interested in a better handling rifle than the one I have now (rear mounted 13 ci tank QB78). I really like the look of Bobs 79 with ninja tank and custom stock. Something like that (custom being butchered original, bondoed and painted) with a slightly longer air tube, slightly shortened barrel and short LDC might make me happy. So many options and so little time. I see why these little QB’s can be so addictive.

 Jason, I’ve seen that picture of the rear sprung valve. Makes me wonder why all valves are not manufactured this way in limited plenum rifles. Have you tested that valve? My concern would be the integrity  and longevity of that little e clip attached to a shaft that gets whacked and whacked and whacked all day long. I considered that mod in my 78 but opted for milling groves in the spring housing like most folks. Worked great for the 78 but I see obvious advantages  to the valve in your picture in a 79. Your mod indexing the spring off a hole in the tank block seems equally effective though and probably less potential for failure. So what are your thoughts on that valve?
Troy

[rsterne]

About 7cc per inch.... There is roughly 6" difference in the length....

Bob

[nervoustrigger]

Jason, I’ve seen that picture of the rear sprung valve. Makes me wonder why all valves are not manufactured this way in limited plenum rifles. Have you tested that valve? My concern would be the integrity  and longevity of that little e clip attached to a shaft that gets whacked and whacked and whacked all day long. I considered that mod in my 78 but opted for milling groves in the spring housing like most folks.

The E clip isn't going anywhere short of reaching in with a tool and prying it off.  Axial impacts from the hammer certainly aren't going to cause it to pop off.  If the hammer were to drive the little spring to coil bind, perhaps it could dislodge but that'can't happen with the valve backer block limiting the maximum lift.

[wll2506]

Jason, I’ve seen that picture of the rear sprung valve. Makes me wonder why all valves are not manufactured this way in limited plenum rifles. Have you tested that valve? My concern would be the integrity  and longevity of that little e clip attached to a shaft that gets whacked and whacked and whacked all day long. I considered that mod in my 78 but opted for milling groves in the spring housing like most folks.

The E clip isn't going anywhere short of reaching in with a tool and prying it off.  Axial impacts from the hammer certainly aren't going to cause it to pop off.  If the hammer were to drive the little spring to coil bind, perhaps it could dislodge but that'can't happen with the valve backer block limiting the maximum lift.

Jason, I assume the e-clip sits in a groove that you had to mill out? Also I assume the plug has to be drilled out so the spring and e-clip go inside it?

wll2506

[Austringer]

The more I look at that valve, the more I like it. Do you believe drilling out the plug to accept the OD of the spring would reduce axial support of the valve stem and contribute to wear in the bearing surface of valve body and possible valve seal alignment issues. Are there many of these valves in service? Definitely not picking on it as it looks so simple I can't quire figure out why there are not more designed just like that (maybe there are?). I see you used this valve in a 78, has anyone used it in a 79?

By the way guys. Thank you for contributing to recent insomnia issues. Beats thinking about work all night!

Troy