More hpa qb79 help

[Trent762]

So i had a problem with my reg on my ninja bottle but fixed it now after taking care of that my 79 in .22 is shooting very bad im running 1100 psi on the reg have slots milled in the valve cup for more air upped the transfer ports to 11/64 im also using 1/4 od ice maker line for tp and added a extra screw to help hold the barrel all springs are stock and i even removed the shroud i also have a breech riser its still not shooting good ill get 2 or 3 in the same hole and the next one will be a inch low or 2 inches left or right iv re crowned the barrel and de burred everything also polished the throat?(where the pellet first touches the rifling) iv tested it to the best i can and its not leaking when fired today i changed the orings on the barrel and bolt and still nothing. Any help is greatly appreciated

[Mr. Panther]

That flier might be because of the pellets your using. Maybe Nervoustrigger or Bob might want to chime in on this. They know more about the 79/78 base QB's than I have forgot.

[nervoustrigger]

It sounds like you are looking at the right things.  A few questions...

Has this barrel ever grouped well?  Or in other words were you working on the crown and leade in hopes of fixing it or in hopes of improving it?

What pellets have you tried?

What does a string of at least 10 shots look like over a chronograph?

Try it without the shroud if you haven’t already.  You may be getting marginal clipping of the baffles.

Lastly I would try it without the riser and go back to using the barrel band, at least as a test to establish a baseline.

[Trent762]

See thats what i was thinking at first so i changed from my normal 14.3 cphp to air arms field heavy and it still shot bad so i tried some different h&n and jsb but still nothing its really weird to because i was getting about .75 5 shot groups at 35 yards with cphps i did test the velocity last night and its still perfect 20 shots all within 12 fps and about half that was within 5 fps so im thinking its got to be something with the barrel or maybe the bolt is messing the pellet skirt up.

[nervoustrigger]

In that case refer back to my edited post...the last two items about the shroud and riser/barrel band.

One more question, were the good groups with CPHP before or after the barrel work?

[Trent762]

It was a dream to shoot last week almost everything i put through it was around 1 inch out to 40 yards i just re crowded it because there was a tiny burr that was driving me crazy and it helped with accuracy some getting under a inch right after.as for pellets you name it iv tried cphp pointed and domed crosmans h&n barracudas match and reg in both head sizes aa field heavy and the 16gr ones and iv tried 13.43 14.35 15.89 18.13 and 25.39s from jsb and i even cleaned the barrel.

[Trent762]

They were good before but better after before my best was 0.90 inch and after i got 0.53

[nervoustrigger]

Taking all that into account, something with the shroud or in the barrel retention are the most likely culprits. 

[Trent762]

Thats what im thinking i pulled the shroud off and its still acting crazy and i did the second screw mod when i first got it so i dont believe thats it and everything is nice and tight with thread locker on it

[Ribbonstone]

Quick bubba-test.

Riser breech and shroud….with shroud now removed...so am guessing no barrel band.

It might need one, but you could test that theory with a thick shim (something semi-soft, not too tight, that contacts both the tube and the barrel).

Even if it is a negative change, that’s still information you can use.

[PG in San Diego]

Hmm, like the others said, I am leaning towards it being the riser is causing the problem. Was the riser and hammer pins homemade? The pin could be dragging.My second thought would be the barrel band. Ribbonstone made a great suggestion. Shim the barrel to the bottle or tube to rule out the barrel band.

[Trent762]

The breech riser was 3d printed and seems to be pretty nicely made but i pulled the shroud and riser off and its still the same as for accuracy also tried a shim as well as the barrel band with and without aluminum tape around the barrel and still nothing i enjoy this kinda stuff but this is getting the best of me im about to pull my scope and bottle off and wrap this 79 around a tree.

[nervoustrigger]

Getting into last ditch territory.  Let’s confirm pellets aren’t getting damaged as they are chambered.  It’s a long shot given you were getting good results last week but if any relationships between key parts (bolt, receiver, or barrel) have changed even slightly, the problem could arise covertly.  How might that happen?  In the course of tightening or loosening fasteners like the barrel grub screws, the M6 bolt holding the receiver to the air tube, etc.  Because the QB is cock-on-close, you’re simultaneously fighting the hammer spring as you’re seating the pellet so it’s almost impossible to detect hard loading.
 
The idea is to seat a few pellets without having to overcome the force of compressing the hammer spring.  How do you do that?  Simply keep the trigger pulled as you close the bolt.  Close it slowly and gently so you can feel how much resistance there is as the pellet is seated…firstly as it passes the barrel port and then as it moves into the rifling.  Any kind of hitch you feel is potential cause for concern.  You’re also looking for consistency…for a given make/model of pellet, you should be able to chamber 10 of them and them all feel substantially the same.
 
As you seat each pellet and note the feel, go ahead and withdraw the bolt again and close it normally to cock it, then go ahead and fire the pellet before repeating with the next pellet.

[PG in San Diego]

The breech riser was 3d printed and seems to be pretty nicely made but i pulled the shroud and riser off and its still the same as for accuracy also tried a shim as well as the barrel band with and without aluminum tape around the barrel and still nothing i enjoy this kinda stuff but this is getting the best of me im about to pull my scope and bottle off and wrap this 79 around a tree.

What are you using to replace the OEM hammer and trigger pins?

[Trent762]

I tried the pellet thing and the head of the bolt is putting a tiny cut in the skirts always on the same side so i polished it off and tested again no cutting still no big improvement in accuracy even without the breech riser i also pushed some down the barrel and its a pretty smooth transition to the rifling but not perfect so im gonna play with that in the morning and completely take apart and clean everything again. As for the cocking pins im using some o1 tool steel i got for knife handle pins dont remember the size but it was just a hair bigger it slides in but wont fall out under its own weight.

[Trent762]

Did a full gut and cleaning replaced some orings and took it out and shot it at 25 yards and i got a beautiful group that was well under half a inch the velocity for the 10 shots was running a little hotter for some reason about 830 but thats fine with a low of 826 and high of 834 but 7 out of 10 shots was 829 to 831 then i went to shoot paper again and i was getting random flyers even with other pellets im so lost on this project haha i believe im gonna polish the chamber?and see if that helps because i was feeling a little rough spot when i pushed some in with any luck thats the problem.

[Trent762]

I got it shooting again shes doing about half inch at 30 now i wanted to say thank you to everyone for all the help i really appreciate it. Not sure what did it but somewhere around the full rebuild swapping all oings new tp polishing chamber?(guess thats what we call it)and running some bore paste through the barrel it works.

[nervoustrigger]

Great, glad to hear it!  Your perseverance paid off, well done!