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Thread: QB78 help

  1. #1
    Join Date
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    QB78 help

    Hey guys

    Hope you're all well and all!

    I've been having some problem lately getting a grouping from my QB78DL :S It used to shoot sub 1'' groups at 30 yards and just lately it's just been doing absolutely rubbish. Seals are all good as i've changed them all but no improvement. Using the same AA field that gave me the good groups. Had a silencer clipping problem but that was fixed, the adapter had been shifted out of line in transit.

    Could it be a crown problem? Valve problem?

    Have re-polished internals to make sure they are smooth and no problem there but the groups are honestly about 6'' at 15 yards

    Any help appreciated and i don't have access to a Chrono anymore so no 'tampering' that could improve power can be done really. Also the thread on the underside screw for the valve assembly has gone so that can't be removed not by me anyway i am ill equiped.

    Cheers and all the best,
    Lewis
    Air Arms S400 Carbine .177 - JSR 3-9x40IR p/x

    QB78 Deluxe .177 - Simmons 8 point 3-9x40

  2. #2
    Hsing-ee's Avatar
    Hsing-ee is offline may also be employed in conjunction with a drawn reciprocation dingle arm, to reduce sinusoidal repleneration
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    Sounds obvious, but is the barrel seated properly in the action and is the retaining stud done up tight? 6" groups at 15 yards means that something is flapping in the breeze! I had a BSA Firebird that wouldn't group, just a tiny bit of re-seating of the barrel and it was spot-on. The pellets were being shaved by the loading block.

    Have a look at the crown with a magnifying glass and see if you can spot any damage. But you would have noticed if you had damaged it surely? Try shooting it without the silencer just to make sure it isnt still a problem.

    Other than that, is the barrel clean? Has it got some gunk stuck in it?

    all the best with it.

  3. #3
    BeerJam is offline Loser of the 2011 'Barn Door Trophy'.
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    Hi Lewis,

    Much the same as has been said by Hsing-ee. Try the easy stuff first, check for any looseness at the barrel mounts and make sure that the barrel band is not too tight. Then take the silencer off and test. If that does not sort it give the barrel a clean and inspect the crown for damage. This can be done with a cotton bud by inserting it into the muzzle and slowly pulling it out, any burrs will catch a few fibres.

    After that you really need to put it over a chrono to check the shot to shot consistency. If you were nearer you could borrow mine.

    With the valve screw issue, it would be silly to start on the internals without trying the above first.

    Good luck, I hope you get it sorted

    Cheers, Mark
    Proud to be a member of; MVAC and Countryside Alliance
    "...... I'm good with the science, but rubbish with the gun "

  4. #4
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    Hey guys

    Thanks a lot for the replies i really appreciate it! It's stuff i've already done but no harm checking again i suppose! If i'm still having problems i shall be posting right back here!!

    All the best,
    Lewis
    Air Arms S400 Carbine .177 - JSR 3-9x40IR p/x

    QB78 Deluxe .177 - Simmons 8 point 3-9x40

  5. #5
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    Is the scope OK? try swapping it over. Does the barrel need a clean?

    Gus
    The ox is slow, but the earth is patient.

  6. #6
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    Quote Originally Posted by Gwylan View Post
    Is the scope OK? try swapping it over. Does the barrel need a clean?

    Gus
    Hey Gus

    Same result with my Simmons 8-point and JSR scope AO unfortunately. I'm cleaning the barrel and checking it over later in the week when i've got more time to do it properly.

    All the best,
    Lewis
    Air Arms S400 Carbine .177 - JSR 3-9x40IR p/x

    QB78 Deluxe .177 - Simmons 8 point 3-9x40

  7. #7
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    I understand your problem accessing valve, but this showing all the signs of a valve stem head that's come loose.
    Weihrauch B&C HW95K, B&C HW35K, BAM B26-2 venoman custom, QB78 DL custom.
    You ain't so big, you jus' tall, tha's all.
    http://ukchineseairgunforum.myfreeforum.org/index.php

  8. #8
    BeerJam is offline Loser of the 2011 'Barn Door Trophy'.
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    Quote Originally Posted by uk litehammer View Post
    I understand your problem accessing valve, but this showing all the signs of a valve stem head that's come loose.
    If that is the case, I have successfully replaced a valve stem assembly without complete disassembly. So long as the valve body seal is in good order, the valve internals can be withdrawn and replaced with access from the filler cap .

    For any sceptics out there, this repair lasted many months and was still good when the next strip down was performed .

    Cheers, Mark
    Proud to be a member of; MVAC and Countryside Alliance
    "...... I'm good with the science, but rubbish with the gun "

  9. #9
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    Anyone got a favourite guide to stripping these guns? I cant resist tinkering.
    Cheers,
    Halam

  10. #10
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    It may seem obvious, but make sure you are not resting the gun on the barrel. This will definitely affect your grouping (drove me nuts BTW).

    If you have a modified pin-type probe, it is possible that as the bolt wears, it puts more sideways pressure on the pellet as you push it in. This can deform the skirt of thin-skirted pellets like the AA-Field and the pellet then fails to grip the rifling. Thick skirted pellets do better in this situation, like FTT or Bisley LRG.

    Finally, if the gun has gone over-power, you will get shot-gun like groupings. I think this is because of the relatively fast twist-rate of these barrels, which were originally designed for lower power. I suggest you take off the silencer and measure the power with a chronograph.

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