Pellet behaviour

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  • #1955
    psion1962
    Participant

    Hey gents a couple or 3 trick questions.
    air rifle is a .20 with a highflo tank and a 20″ barrel, Bottle filled to 2900psi
    Wheel setting 3:1

    Firstly I cronied about 35 shots with the following results:

    fps
    945
    934
    947
    961
    963
    968
    970
    963
    974
    978
    976
    974
    980
    993
    992
    990
    983
    995
    993
    996
    990
    1000
    1000
    1002
    999
    998
    993
    1004
    1010
    1013
    1011
    1018
    1010
    1011
    1016

    I got a steady increase in speed until 1000 fps. pretty sure it will drop off again? The only thing I can think of is that the pressure in the tank closes the valve early and as the pressure reduces the valve performs properly. Will try filling the bottle to 2700psi and doing the exercise again

    Secondly
    I found the grouping with jsb’s at these velocities to be brilliant (1 inch at 65 metres), but the predators were all over the place. I was getting a 3 inch group. I could see the pellet spiral at the target. Could the velocities be too high for these pellets? The grouping at 45 metres was good.

    Lastly

    how do I calculate the velocity of the pellet down the line? (at 20, 30, 40 metres etc).

    thanx guys

    dave S

    #37759
    crimson
    Participant

    Uhhh… so, what you said you’re selling?

    😛

    About your questions, someone better than me will help you. About the Predator, maybe your gun just doesn’t like it…

    #37764
    sotxguy
    Participant

    Can’t help you with the velocity curve you’re having. Hopefully a tuner will chime in with an answer

    I have s stock TSS and JSB predators are accurate out to 35~40 yrds then start to get wild.

    To measure down range velocity, put the Chrony downrange and use something to protect the face/chassis. If you do hit it, the damage will be limited to a rod or diffuser. BTW you won’t be the first to shoot a Chrony
    😯

    #37766
    tyke
    Participant

    Hi Mate
    The higher pressure when you filled the bottle isn’t allowing the valve to knock open fully the power will increase when you reach the sweet spot. Try filling the tank to different lesser pressures until you see you are straight on the sweet spot or plateau. Not sure about the conversion but my Stealth hits the sweet spot at 180bar.

    #37768
    riffraff
    Participant

    Are you shooting through a shroud?

    With every shroud Ive built, I always find one or two pellets that wont shoot straight.

    I never see any signs of clipping, but I think the air-pressure dynamics inside the shroud are forcing pellets a little out of stabilization.

    I saw the exact thing you describe with predators with my first shroud. With the next shroud they shot fine.

    #37769
    psion1962
    Participant
    quote :

    Posted: Mon Apr 14, 2008 9:10 pm Post subject:

    ——————————————————————————–

    Are you shooting through a shroud

    Yip, maybe time to try a different shroud
    and reduce the fill pressure. will try 2600 psi

    thanx gents

    #37770
    riffraff
    Participant

    I doubt fill pressure will have any effect.

    Predators shoot pretty good even supersonic.

    Good luck with the shroud building, its part of the fun IMO.

    #37773
    colinb
    Participant

    need a heavyer hammer and spring for that valve/press combo to get most out of it

    #37793
    tyke
    Participant

    Over filling your air bottle /cylinder will give you less power until you reach the plateau point, its more obvious with cylinder rifles i.e. Air Arms S400 with a lower fill capacity but it is the same effect with bottles as well, you’ve proven it with your chrono results.

    If you are shooting through a shroud/silencer or are going to build one I have found to eliminate shroud turbulence the internal spacer are best cut with a cone shape facing towards the incoming pellet. The cone or cones strip the turbulent air from around the pellet increasing stability and accuracy, they also reduce the noise.

    #37798
    bodhisdad
    Participant

    I’d play with the tophat and turn it in a 1/4 revolution and reshoot the string. What you have is a pretty good shot spread from shot to shot, if that can be tightened up it will help. Its good that you have a bell shaped curve, it will yield more shots per fill and a better overall avg. of extreme spread from shot to shot. Starting lower then a climb and back down is preferable to a straight desent. What your getting .20cal is what alot of us would like to see in .22. say you take 40 shots starting at 950fps up to 1000fps and back down to 950fps. You have a spread of only 50fps and 40 shots. You would never see this tight of a spread if you start at the most extreme velocity and head straight down in velocity. The extreme spread would be double. I think a little fine tuning of the tophat and you got a really consistant AG and some numbers i would like to see. Fill to the same pressure you did for the string you posted. Unless you want topend velocity and downhill from there 😕 In a perfect world i’d want to be starting at 925fps up to 975fps and back down. That would be a awesome spread and the perfect speed for Kodiaks. Filling at a lower pressure is in essence eliminating the upper end of your bell curve, that doesn’t make sense to me. If others are doing this, maybe they should be asking why? You’ve got a good thing going, just fine tune the numbers.
    So for answer 1. play with your tophat and tighten up the numbers a bit, your better then good.
    2. ODD, i would expect the preds to be more stable at greater speed then the JSBs. You might have a odd ball barrel, clipping? Did you change your grip on the AG? There should be a variable.
    3. Check out Chairgun.com, i believe with the known muzzle velocity and BC of the pellet you can calculate the velocity at distance. I haven’t looked at that program in quite awhile. The other option is place the chrony downrange and protect the screen of it with some steel plate.

    #37799
    bodhisdad
    Participant

    I crunched the numbers and it is pretty dam good. Other then maybe very fine tuning, trying to get your starting point at 925fps this is what your numbers tell me. Note that the higher you go on your shot count the better the extreme spread is.

    shot 1-10= 960fps avg. ex. sp. is 4.4fps
    shot 1-20= 973fps avg. ex. sp. is 3.1fps
    shot 1-30= 982fps avg. ex. sp. is 2.6fps
    shot 1-35= 987fps avg. ex. sp. is 2.4fps

    I’d like to see what you get if you shoot the gun back down to 934fps.

    #37812
    psion1962
    Participant

    Thanx for the input
    I will fill up to 2900 psi and shoot a string until I get back to 940-ish and post the results. Where do I get a copy of the shroud design. I would like to get someone in sa to make one.

    regards

    Psion

    #37815
    bodhisdad
    Participant

    I look forward to the results, those are really good numbers. The consistency is pretty good after you crunch the numbers. Doesn’t look that way, but the calculator doesn’t lie. The more you shot the better the numbers got. If after the peak the consistency stayed the same it would be alot of shots and a really close spread. Alot (most) of strings are full power then down hill, you get more shots and a better spread if you can get a bell shaped curve like you will havew on the other side of 35 shots.

    I have a write up in the shroud area as to how to build the shroud and what to put in it. I’d use a fender washer instead of the felt ones. Found out today after 20 shots that the felt moves around and can’t take the energy. 😳

    #37816
    savagesam
    Participant

    Understand these questions is from a newbie stand point. Is your gun stock? What are the weight of the pellets? I had no idea you could be around 1000FPS with the P.W. at 3-1???. Stock top hat setting? Does adjusting the top hat void your warranty? I understand what you mean by the bell curve and couldn’t agree more. Those are some sweet numbers. Can’t wait till my chrony gets here Friday. Is it a talon or condor?

    #37820
    psion1962
    Participant
    quote :

    Understand these questions is from a newbie stand point. Is your gun stock? What are the weight of the pellets? I had no idea you could be around 1000FPS with the P.W. at 3-1???. Stock top hat setting? Does adjusting the top hat void your warranty? I understand what you mean by the bell curve and couldn’t agree more. Those are some sweet numbers. Can’t wait till my chrony gets here Friday. Is it a talon or condor?

    I have a 20 inch .20 talon, with a highflo tank and valve along with a heavy hammer. the setup is designed for enjin pellets (23 grs) primary use is rabbit and geese (who are as tough as nails) The enjin does about 900 fps. With the JSB’s (14 grs) i get 1090 – 1100 fps at a setting of 10:1. Any higher and The entire tank dumps. The setting of 3:1 seems to be a nice balance between power and consistancy (970 fps). With lower setting I get very erractic shots. I don’t think the main spring is strong enough to keep the valve open.
    I am going to reduce the top hat spacing to get the speed down to 950 fps with the JSB’s. The enjins at that speed don’t stop rabbits. It doesn’t deform and passes straight through. The Jsb work a treat. I recon if I can get the accuracy right for the predators they will be real stoppers on those rabbits. Tests on pheasant size birds are very promising. The jsb give me a 1 inch group at 65 metres, need that from the Predators.

    cheers

    Psion

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