Velocity drift for Challenge HFT - any idea how to resolve?

Discussion in 'Steyr' started by astra, May 29, 2019.

  1. astra

    astra New Member

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    Two months ago I became happy owner of new Challenge HFT.
    Then I set the velocity at 238m/s as it gave me the best consistensy both at 50m. on paper target and at chrono test +/-0.5m/s
    For two months I have shot 1500 pellets/3 tins.
    Yesterday at the range at 50m on paper I noticed a wide spread of the group inspite of using pellets from the same batch. I checked the velocity and it was drifted up to 242m/s . I lowered it down to 238m/s and the group became as tight as it previously was.
    I measure the velocity always at constant environment after having the rifle tempered for some time so I do not suppose a temperature issue.

    My question is about the significant velocity drift after shooting just 3 tins.
    Is this drift normal or it is related just to the brand new rifles until they settle down?
    Do I need periodically to check the velocity if it will drift again and again?
    Drift of 3-4m/s has quite an impact on accuracy on 50 meters and can you give me advice how to keep it constant to the preset value of 238m/s for longer period - shots count?

    Thank you!
     
  2. broekzwans

    broekzwans New Member

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    I think it's just the settling of the gun. But to be sure just keep monitoring it every now and then :)

    I wouldn't worry about anything being wrong at the moment ;)
     
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  3. Ash Bailey

    Ash Bailey Would-be HFT shooter.

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    What pellets do you use? A variation in pellet weight will result in a variation in muzzle velocity.

    Not all 8.44gn pellets weigh 8.44gn!

    Try measuring muzzle velocity with a batch of weighed pellets. My Challenge gives a variation of around 3 fps/1 mps with pellets of the same weight.

    Hope this helps.
     
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  4. astra

    astra New Member

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    JSB Exact as recommended being the most stable in wind.
    I also tried chrono test with RWS R10 0.45grams for pistol as I have got these. Same random velocity peaks of +5m/s around the steady value of 260m/s.
    Measuring weight is what I will do as a first measure to resolve the velocity peaks and shifts. I suppose it is not only the weight but the diameter variations that I'm unable to measure and select.
    However will proceed step by step.
    Thanks for supporting!
     
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  5. astra

    astra New Member

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    It seems I have found what was the cause of the velocity drift.
    It is the temperature that causes the drift but in a strange way. I normally expect the higher the tempereture the higher the muzzle velocity but in fact it is exactly the opposite!!! :D
    Today I performed the following experiment.
    I set the velocity to 237m/s at room temp (25degree). Then I put my Challenge HFT under the summer sun for more than an hour until it became unbearably hot. Measuring the speed gave me 234-235m/s and no higher velocity results no matter the shots count... I was stunned! I expected much higher speeds. :D
    Just after this I put the rifle at the air condition outlet and cooled it down to 15-16 degrees for half an hour. It became really cold! The chronotest gave me results of 240-241m/s.
    Exactly the opposite of what I expected but no doubt of the results :D
    Obviously, the velocity setting of around 237m/s outdoors in the Summer and checking the speed at home gives this shift towards higher speeds.
    What is funny - I still strugle to accept the negative dependance of the velocity against the temperature :)

    Anybody have seen the same or the opposite temperature behaviour of his Steyr FT rifle?
     
  6. hmangphilly

    hmangphilly Can't Re Member

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    Next step would be measure the reg pressure at various temperatures
     
  7. astra

    astra New Member

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    Why should I measure the pressure over the temp? What useful info this could bring to me?
     
  8. garym

    garym Active Member

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    Astra, in our extreme temperatures you not only test the rifles temp. but also make sure the pellets are subject to the same temperature variances. Just another variable of testing, expansion and contraction of all components in the chain.
    The reg. pressure at different temps is your flexibility, adjustable. Hence the need the to monitor the reg.

    At the Bulgarian open, my regulator pressure dropped, rifle still had all anti tampers fitted. I was doomed. ;)
    Other Bulgarians just regulated the speed via the regulator, ie, turned up reg to deliver the required speed. All part of the practice area testing. Knowing temp variances that can be overcome by tweeking the regulator is a bonus.

    I am happy to be shot down or corrected.

    Happy shooting
     
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  9. astra

    astra New Member

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    Hi, Gary! Thanks for reply.
    I did investigate the velocity over temp more in depth and repeated this temp span twice just to be sure (in two sequential days, starting from the aircondition outlets and finishing under the Summer Bulgarian sun).
    It appears that from 15 degs up to 35 degs (a temp range of 20 degrees that happen to appear in a normal FT day in Bulgaria) the average figures of velocity change is from 242m/s @15deg down to 236m/s @35deg.
    Is this a lot?
    The balistic calculator says that @50 meters the maximum vertical shift is 3-4 clicks (1/8 MOA) or just 5mm. Seems neglectible and no worth to worry and tweak the Steyr.
    In fact, the devil is in the details :( :( :(
    The Challenge FT barrel is quite accurate @speeds between 237 and 239 m/s. Outside this limits the grouping becomes spreading! Easy seen even @25meters distance... I observed that the 450mm long barrels that Steyr fit to their Challenges are quite stringent towards ... everything :( It is hard to keep such a narrow velocity limits over a Summer day temperature span!
    Moreover - it is hard to find suitable pellets series for this barrel. In fact I have found only one serial number of JSBs that perfectly groups @50 meters and keep it as a family treasure :) All other series and tins that purchased simply do not group but spread. What is the benefit of such a barrel even if it's a LW made???
    I asked Styer with a related question and will see what would be their advice if any...
     
  10. garym

    garym Active Member

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    You have already answered your own questions mate, welcome to the world of FT and temp variances. Pellets, that is a separate topic, my FTP 900 and my mates Anchultz 9015 both like batch 70 and 46 from this years JSB batch. Petko supplied the pellets.;)
    I have little knowledge of Styer preferences. Again, testing is your guage. If you want some pellet samples to try, drop me a PM.
    Gary
    PS. Sometimes us engineers tend to analyse things too much in FT. Trust me, sometimes over analysing epecially with the 'work head' on will cause mental conflict.
    A poster on this forum, once told me " I do not know how it works, but it does". (Thanks) A important and valid lesson for me. Trust others experience, enjoy the learning mate.
     
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  11. Petko Dishkelev

    Petko Dishkelev New Member

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    It is time for you to test JSB 4,53 pellets.
    Maybe your barrel will likes them more then 4,52.
    But try different speeds again, to see how it is with the vibration of the barrel.
    The compensator may also need replacement.
     
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  12. astra

    astra New Member

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    As a matter of fact I went on my tests with another series of JSB 4,52mm - the 75 series. It was quite surprising that the velocity change over the temp had narrowed significantly!
    It started with 242m/s @15 degrees and ended with 239m/s @35 degrees. Just the JSB serial number had been changed and temp drift became neglectible...
    Now the question is: Who to blame? Steyr (not anymore I guess), JSB or ... :)
    I must say that the 75 JSB series produced quite consistent velocity results!
    Moreover I shoot them yesterday in moderate, variable wind - the pellets needed very low windage correction - no more than one mildot x40 zoom at 50 meters.
    Obviously there is quite a differnce between the batches of JSB pellets...
     
    Last edited: Aug 16, 2019 at 1:46 PM
  13. Petko Dishkelev

    Petko Dishkelev New Member

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    And the last is the replacement of this capricious barrel.
     
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  14. astra

    astra New Member

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    I intend to shoot some more 1000 - 1500 pellets through this reverend LW barrel just give it the chance to get into the right condition.
    Then will proceed with the step you mentioned. Barrel replacement should not be a painful exercise :)
     
  15. Simonix

    Simonix Member

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    Interestingly, I spoke to the previous owner of my Challenge HFT. He said it’s preferred diet was the JSB 4.53.

    I went and bought a sleeve of them, only to find out that for some reason, I couldn’t get it to group beyond 30 yards!

    Ended up testing a load of different head sizes and makes, and it seems to love the H&N Barracuda FT 4.51?!

    My first rifle that doesn’t seem to like JSB....:eek:
     
  16. astra

    astra New Member

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    I joked few posts above that the only serial number of JSB tins that group @50 meters simply in one hole has been given a statute of a family treasure! :)
    No shooting with them pellets but just contemplating with honour. Who knows - this might be the first and the last suitable pellet series for my extremely exact LW barrel ;)
     
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  17. Petko Dishkelev

    Petko Dishkelev New Member

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    The different shapes of the forming matrices have an impact, but each series also has different speeds that make them stable up to 50m. You have convinced yourself of this by your tests.
    The tolerances and the formation of the cuts in the barrel itself are also essential. I exclude the crowns that are supposed to be in order.
    The Russians' favorite "polishing" in moderation sometimes also gives the expected results.
     

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