Stock bedding ???? Poi ????

Discussion in 'Piston & Spring' started by madplinker, Jan 4, 2020.

  1. Nomads HFT

    Nomads HFT Well-Known Member

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    I take it this is the rifle with a 'skirtless' piston, David? If so, then unless we can establish whether it has the standard piston seal. an aftermarket alternative, an o ring, or a combination of o ring and seal, we can only guess at the cause of the POI shift.
     
  2. Adam

    Adam Well-Known Member

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    I've had this exact issue with:
    • LGU standard action 3.2mm TP
    • LGU Tony Leach skirtless 25mm piston (Sized seal backed with o-ring)
    • 97K standard HW seal + v-mach spring
    I can't remember if I had it with the 97K on the V-mach seal.

    The magnitude of the shift was larger with the skirtless piston, but with the standard ones it was still enough to bring the pellet out of kill at 12 or 6 o'clock at 50 yards so it's academic really. I put the larger shift down to the lighter piston being more sensitive to frictional losses.

    I'm more or less in agreement with Rob as to the cause, however I acknowledge there are other causes of similar shifts. I have in the past also experienced shifts due to stock bolts coming loose, and I'm still of the opinion there's a cyclical effect of rotating pistons working their way into a position where the friction is increased, then going past that position. It's therefore possible to get combinations of these where you've no bloody idea what's going on! :D
     
  3. madplinker

    madplinker Active Member

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    Hi
    Yes Jim the piston is skirtless its not a o ring seal for sure that I do know.
    I am surprised that this happens quite a lot I thought this would be quite rare.
    There is a few things I want to try probably the first one just try the action in a standard stock and see how that works out. The stock May have some inletting issues.
    Second thing would change to a standard type piston and change seal etc.
    Third phone a priest and exercise the gremlins

    The most annoying thing in a way is when no gremlins and everything is working has it should be it is a very accurate rifle and has loads of potential in the set up it is in.
    There has been lgu horror 97 same any tx 200 with same issues ????
     
  4. Adam

    Adam Well-Known Member

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    @Alanok2002 has had it with a 22mm TX
     
  5. Nomads HFT

    Nomads HFT Well-Known Member

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    One or two, David :D

    Problem is, everyone is looking for a single simplified reason for a single symptom (namely, vertical POI shift) that can manifest itself under a wide range of conditions that, in my opinion, could (will) have more than one cause.

    Cold weather POI shift between the zero range and course is unquestionably thermal expansion rates of steel and synthetic piston seals altering the point in the recoil cycle that the pellet exits the muzzle. Other vertical POI shifts at fairly benign temperature we've identified as being down to accumulated pellet debris on the breech seal in a rifle which, I may add, went through a full four seasons of HFT competition without needing so much as a single sight alteration. Other reported POI shifts are probably due to (dieseling) fuel depletion, or evaporative losses of lubricant fumes during the shot cycle thickening the remaining cylinder lubricant, either of which may or may not alter the muzzle velocity, but which will shift the pellet exit within the shot cycle.

    I doubt that swapping the stock will change things, but it's worth trying just to rule it out.
     
    Last edited: Jan 7, 2020
  6. FPoole

    FPoole Active Member

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    Scratch my theory, I just had the overly tight seal pull off the piston on the sight in line. Luckily I walked a couple hundred feet, up to my house, and fetched a Mark1 TX200 to shoot with.:D
     

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