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Thread: More on Predators

  1. #1
    Senior Member Doc Sharptail's Avatar
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    More on Predators

    Decided to take a break tonight, and sit and shoot the 20-78 from rest.



    JSB Predators @ 12 Meters, 6X on the 4-12X Legend. 5 Shots @ 1800 PSI, which out of this rifle is the mid 7's.

    Upping the pressure to 2000 PSI in this gun reduces velocity by almost 100 fps for the front third of the string:



    5 shots again, and the reduction in group size becomes apparent here.

    I'm not really sure what's happening here. I had noticed similar behaviour with velocity changes in my riflized 13-XX's. From what I've seen, tight groups with the predators are not static to set velocity- going up another extra 100 fps seems to tighten groups up again...

    Just for general interest:





    Two views of a recovered Predator after driving the pull ring tab and it's rivet through a soda can 740-760 fps.



    The bottom of the can, I sort of felt sorry for.

    What's funny is that this sine wave of group sizes doesn't appear so glaringly with the JSB Exact Heavy, which is a favourite of mine...

    Regards,

    Doc Sharptail
    Last edited by Doc Sharptail; Apr 03 2013 at 02:59 AM.
    "Ain't No Half Way"

    -S.R.V.

  2. #2
    Senior Member sholo's Avatar
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    Nice groups, Doc! Tree rats beware!!

    I love my P-mags too. I have my 2260 HPA (regulated to 1500 psi) tuned to shoot them @ 920-925 fps and 10m groups pretty much mirror your 2000 psi groups, yet if I turn the RVA in a couple more turns (955-960 fps) they open up to your 1800 psi group size (maybe even a tad larger). Me thinks 900 fps (or slightly over) is about the upper limit for accuracy with these - at least out of my 24" Crosman barrel anyways.

    What's funny is that this sine wave of group sizes doesn't appear so glaringly with the JSB Exact Heavy, which is a favourite of mine...
    I wonder if that's because of the extra weight of the Heavies' and domed head design? They do have a much better BC than the Preds'...I really gotta get some 18.1 gr JSB's to try out. Have tried just about every pellet on the market except those!

    Too bad you're a province away, would love to go on a squirrel hunt with ya!


    Todd
    Last edited by sholo; Apr 03 2013 at 03:53 AM.
    Todd

    Don't sweat the petty things, and don't pet the sweatty things!

  3. #3
    Moderator rsterne's Avatar
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    I'm betting your groups sizes are related more to barrel harmonics than velocity per se.... and I'm gonna bet Sean will chime in on that.... If that is the case, a very small muzzle weight (or changing the weight if you have one) may well alter the velocity at which the tightest group occurs.... On a whippy Crosman barrel I have found a 5 gram change is noticable....

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

  4. #4
    Senior Member SeanMP's Avatar
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    It certainly looks barrelish to me.

    I'm seeing a bit of a horizontal tendency....Did you put in the extra setscrews to get rid of the wiggle?

    If you have then yes a few grams of weight should do the trick. Hang an S hook off the barrel with a little pan attached. gradually add a few pellets at a time until your group closes in then weigh the pellets. From there it's easy to make up a pseudo muzzle brake or guard to match the weight.
    Sean

  5. #5
    Senior Member Doc Sharptail's Avatar
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    Quote Originally Posted by SeanMP View Post

    I'm seeing a bit of a horizontal tendency....Did you put in the extra setscrews to get rid of the wiggle?
    I think that's one of the taps I have to get. I'm looking at this job with a bit of trepidation~ run-out on the drill press is getting pretty bad. I definitely don't want ovalized holes in the breech.

    Any in-put on 10 & 2 vs. 3 & 9 set screw positions? 3 & 9 would be easier with a single pass of the drill...

    Regards,

    Doc Sharptail
    "Ain't No Half Way"

    -S.R.V.

  6. #6
    Senior Member SeanMP's Avatar
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    I went with a 2:30-9:30 idea but I don't think it's critical.

    To be honest with a good center punch mark I wouldn't be afraid to do that free hand. But if your concerned I'll send you a drill bushing with the guide holes already drilled through for the price of postage. Then your drill can just follow my holes.
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    Sean

  7. #7
    Senior Member sholo's Avatar
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    Quote Originally Posted by SeanMP View Post
    Hang an S hook off the barrel with a little pan attached. gradually add a few pellets at a time until your group closes in then weigh the pellets. From there it's easy to make up a pseudo muzzle brake or guard to match the weight.
    Cool idea...gonna have to try that one myself!


    Todd
    Todd

    Don't sweat the petty things, and don't pet the sweatty things!

  8. #8
    Senior Member SeanMP's Avatar
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    Quote Originally Posted by sholo View Post
    Cool idea...gonna have to try that one myself!


    Todd
    Ya I'm full of good ideas....amungst other things
    Sean

  9. #9
    Administrator AirGunEric's Avatar
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    Quote Originally Posted by SeanMP View Post
    Then your drill can just follow my holes.
    Sometimes, I'm at a loss for words.



  10. #10
    Senior Member Doc Sharptail's Avatar
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    Quote Originally Posted by sholo View Post




    Too bad you're a province away, would love to go on a squirrel hunt with ya!


    Todd
    I make it a habit to never say "never". It'd probably be a 2 day drive- lot of ground to cover...
    Filed away in dept. of things to remember....

    Regards,

    Doc Sharptail
    "Ain't No Half Way"

    -S.R.V.

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