What is the usual cause of POI shift on the 800?

Discussion in 'FWB' started by Bo_Nidle, Sep 25, 2018.

  1. Bo_Nidle

    Bo_Nidle Member

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    I recently purchased an alu-frame 800 FT 16j, (used but had been bought new last June by the previous owner who barely used it and has now given up shooting). I knew about the POI shift reputation but the price was too good to pass on.

    I did encounter a shift, normally 1/2 mildot down, 1/4 mil dot left. So in an effort to cure it I placed the sight in a BKL 354 cantilever mount mounted on the front breech block only, but it didn't work. (The previous owner had had the sight mounted on the rear block but had not used the rifle enough times to notice anything).

    I then removed the weight from the front of the barrel, lo and behold the POI immediately returned to where it should be. I put the weight back on and the POI shifted again.

    By moving the weight forwards and backwards I found the POI would shift again to different places. Took it off and the POI corrected again. I repeated this process five times with the same results.

    I have now left it off. It now appears to be behaving itself.

    I somewhat puzzled by this to say the least as the weight seems to have no effect on the rifle other than balance that I can see.

    Anyone know why this would give the effect it did?

    And what other causes are there to watch out for?

    I was also considering having a CF shroud fitted to replace the stainless steel one and allow the barrel to float. Is the shroud epoxied on like the old Dommies?

    Any advice or info appreciated.
     
  2. Conor

    Conor Never been banned from sales Staff Member

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    What’s 1/2 a mil dot and what’s 1/4 of a mil dot?
    Do you have a precise distance and exact measurement of the POI change?
     
  3. Darron

    Darron Dwarf Slayer

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    What is the usual cause of POI shift on the 800?

    Martin Calpin
     
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  4. Bezzer

    Bezzer Member

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    1. Look up barrel harmonics, moving the weight around alters the harmonics/movement and affects the POI. It's "only" an airgun but the barrel still moves about fractionally so the pellet can leave the muzzle at a slightly different barrel angle each shot.
    2. Sell it and make Jane's day:D
     
  5. Nomads HFT

    Nomads HFT Active Member

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    Hanging a mass on a cantilever barrel, or moving a mass on the barrel, alters the frequency of the first mode barrel vibration, which most wrongly refer to as 'harmonics' (barrel vibrations are not integer multiples, and therefore not harmonic). This can cause the pellet exit to move toward an antinode (good for accuracy) or a node (muzzle in the 'at rest' position, not good), and shift POI.
     
  6. eggie

    eggie Active Member

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    THERMAL LIFT.............:)
     
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  7. MTA

    MTA Member

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    Mine's full floated and ISP regulated. Without a weight it produces a distinct double group, about 10 mm vertical spread, at 35 yards. 150 grams of weight, positioned carefully, brings it to a single hole. 9-full.jpg
     
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  8. Bo_Nidle

    Bo_Nidle Member

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    Thanks for the replies chaps.

    CONOR: You're quite right, I should have been a bit more precise:

    Thats shooting at 9 x mag at 25 yards on an SCB2 reticle (24mm Viper Connect).

    It translated to about 1cm down and 5mm right ( or 5mm left with the weight moved rearwards ) if memory serves (I no longer have the target or I'd measure it).

    MTA: Thanks for the info. What make is the riser rail you are using in that photo?

    NOMADS HFT: Thank you for introducing me to some new words. I have looked up the meaning and think I understand...although physics always makes my brain hurt! :confused:

    BEZZER: I suspected it may have something to with resonance, and the Evil One can sod off, I'm not selling it, it won't beat me.
     
  9. MTA

    MTA Member

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    It was made by Jon Harris back in 2014. Mounts on the barrel block only and floats at the rear.
     
  10. Bo_Nidle

    Bo_Nidle Member

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    Thanks for that MTA.
     

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