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  1. #1
    Senior Member Doc Sharptail's Avatar
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    Flat...

    ...string, but no curve. Good news is the nose-dive holds off for 13 or so shots...

    Outdoors, 15 C. 3000 PSI on the 20-78 with the 3K tube.
    I put the max flow valve back into the tube. I also made a spring guide for the stem return spring. I removed all the hammer spring pre-load, with just the stock spring in the gun.
    Shot with the 18.1 gr JSB Exact heavy:

    1.) 743.7
    2.) 747.7
    3.) 746.8
    4.) 740.3
    5.) 742.7
    6.) 735.7
    7.) 738.1
    8.) 746.8
    9.) 744.8
    10.) 739.8
    11.) 735.7
    12.) 731.6
    13.) 722.4
    14.) 722.1
    15.) 713.0
    16.) 704.6

    Going by the gauge on the hill pump, that string used about 1400 PSI.

    I have mixed feelings about this. Frankly, I was hoping for a bit more usable velocity.

    Is continually subjecting the gun to 3K PSI going to be a concern?

    I'm going to leave things as they are for a while.
    It looks like I'm going to have to go for a regulated bottle set-up. I may end up putting the .177 bbl on the 3K tube at a later date...

    Had a pretty good day out there. The off & on rain kind of put a damper on the use of the hill pump. I would have shot the 20-78 a lot more if the weather had been better. Always have a back-up 13-XX pumper a long, so the day did have it's merits. A few pix if anyone's interested...

    Regards,

    Doc Sharptail
    "Ain't No Half Way"

    -S.R.V.

  2. #2
    Moderator rsterne's Avatar
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    The string would indicate you have just a fraction too much hammer strike as you're missing the front part of the curve.... That in itself makes little sense, I would except a stock hammer spring to not even open the valve at 3000 psi.... The velocity seems really low for 3000 psi.... With extensive valve mods I was getting over 900 fps with 18 gr. JSBs at only 1600 psi.... Something strange going on for sure....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  3. #3
    Senior Member Doc Sharptail's Avatar
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    I'm not sure what's happening either, Bob. Before the factory stem packed it in, I was seeing mid to upper 9's at 1600 and above, albeit with the non-string nose dives.

    Tube chills down a lot, shooting from that 3K at a rate of a shot every 45 seconds or so...
    I haven't been able to locate any leaks- on the tube, or the firing cycle.

    I really have to lean on the pump for that last stretch between 2800 and 3000....

    Regards,

    Doc Sharptail
    "Ain't No Half Way"

    -S.R.V.

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