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Pcp Accuracy Waning Around 130 Bar?

Discussion in 'Technical' started by Simonsays, Jun 26, 2020.

  1. Simonsays

    Simonsays Donator

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    I recall being told by Brocock back in the dark days of my shot gunning Compatto that all pcps lose a bit of accuracy (ie groups open up) as the pressure fall past a certain point. I didn't take much notice but it was certainly true of the Compatto - it was even worse below 130bar.

    Now the Compatto has been long replaced by the Kozak which is in a different league. But curiously I have noticed on several occasions on the range that the Kozak goes off song as the pressure approaches 100bar, but comes straight back with more air.

    Can anyone confirm that this is a pcp trait?

    Oddly a chrono check showed normal fps at this pressure which seemed odd as the POI had dropped and groups opened out.

    Whatever, it prompted me to install the Lane reg which was a job long overdue. It seems to be good this time around. Its a dead easy install, didn't even need tools.
     
  2. jesim1

    jesim1 Kit bitch to the Stars

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    Your only correct in that as you get to a specific point with any gun it will start to lose accuracy due to a fall in pressure and therefor fps as it either falls off the reg or starts to lose power with low pressure on an unregged gun. The idea that it will "lose accuracy but retain the same fps performance" is not correct, but probably in your head or due to external condition. Sometimes when your "looking" for a problem you find it :thumb:
     
  3. willo1962

    willo1962 Posting Addict

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    I have shot a full string from my Ultra MMC from 180 bar to 100 bar. It only puts out about 10.5 flb max but at about 50 shots I am down to about 9flb and by 60 shots I am at 100 bar and my POI has dropped about 1cm @ 20m but still grouping well. I could probably still get acceptable groups if I shot to 50 bar as a pistol will group well at that distance with only a few flb
     
  4. Simonsays

    Simonsays Donator

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    I would expect vertical stringing if it were just a matter of losing MV. But it seems to be a general opening of groups.
     
    willo1962 likes this.
  5. Grey Man

    Grey Man Posting Addict

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    I would guess that as power drops, oscillation increases.

    Paging Cloverleaf ...
     
    Simonsays likes this.
  6. robs5230

    robs5230 Oversprung

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    I think Brocock may have been clutching at straws with that response :)

    As far as regged rifles go, in well set up ones, when pressure drops off reg, all I've seen is a drop in velocity and POI.

    In something like the Kozak, where regs and setup at sub 12 are far from optimal, I'd think the reg simplys struggles to cope below a given pressure, and it may well cause inaccuracy.
     
    Simonsays likes this.
  7. Simonsays

    Simonsays Donator

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    Yep, though to my mind it would lead to vertical stringing not shotgunning (relatively speaking).

    Anyway, it’s prompted me to install the replacement reg I bought from Rob Lane a while back. Bit of a faff to stop blow by but I think I’ve gotten there. I’ve posted the story in the technical section.
     
    phil.m and robs5230 like this.
  8. phil.m

    phil.m Donator

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    Was your compatto a regged one? The 1st one I had wasn't regged & that suffered from poor accuracy too.

    I'm afraid I don't know the answer to the problem, but are you comparing apples with oranges here?
    None regged / regged?

    Good luck getting it sorted.
     
  9. cloverleaf

    cloverleaf Super Moderator Staff Member Mod/Admin

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    My guess would be either the cylinder warping / relaxing and pulling the barrel about (if the barrel's not floated) or excessive valve duration (in an un-regged gun) causing greater pressure at the muzzle and associated destabilisation..
     
    Simonsays likes this.
  10. Septic

    Septic Superbus et Vetera

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    Lane regs are great...

    I’ve had no experience with the kozak.. But I can say from many years working with HPA in industry, that some regulators are amazingly accurate at normal design pressures, but become very slightly unstable when the air feed isn’t significantly higher than the desired output... Which might be why you see a loss of accuracy while the fill pressure is still more than you need for the shot...
     
  11. Simonsays

    Simonsays Donator

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    No Compatto not regged. Could indeed be apples and oranges but I was simply tying together two bits of circumstantial evidence.

    I did eventually get a regged Compatto replacement from Brocock which was spot on but in the meantime I’d been seduced by the super compact Kozak, so it got sold.
     
    phil.m likes this.
  12. phil.m

    phil.m Donator

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    I remember your posts well & by pure coincidence (and drooling over airguns) I followed a similar route.

    However I had a kozak from the bad batch, that was a return re-sold by Pellpax.
    So I ended up with a brilliant regged bantam.

    Good luck getting it sorted. Could cloverleaf be on to something? Isn't the barrel banded on the kozak?
     
  13. Simonsays

    Simonsays Donator

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    Yes it is banded but why only around 130 bar?

    Worth investigating.
     

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