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Chronograph readings

Discussion in 'Shooting Related Threads' started by mtjack, May 25, 2012.

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  1. mtjack

    mtjack TS Member

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    In an eairlier thread one of the respondents stated that a hobby cronograph,

    or one of the type avalible to typical handloaders, measured the speed of only

    the first pellet out of the chute, not the center mass of the shot string. He

    also indicated that the difference between the first pellet and the center

    mass would be 50fps. Can anyone else with hands on knowledge corrobrate this?

    I would agree that the speed indicted would be that of whatever pellet breaks

    both optical planes first, and I understand that the shot strings but at what

    distance would the 50fps difference come into play, 10 feet or 50 yards?

    Additionally, based on readings I've recieved from my chronograph 50fps is

    not outside the normal range of velocities. Any rocket scientists out there?

    Jack
     
  2. 5 Stand Dan

    5 Stand Dan Active Member

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    With many years of empirical research, I can state without any reservation, it's the Indian not the arrow.

    P.S. A really good reload will have about 15 +/- fps in a sample of 7 or more. I use a Oehler 35P

    P.S. +1 Does 50 +/- fps matter? Only if you think about it.

    5 Stand Dan
     
  3. AveragEd

    AveragEd Well-Known Member

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    I have a ProChrono and they suggest deducting 35fps from readings with shotguns.

    Ed
     
  4. BigM-Perazzi

    BigM-Perazzi Well-Known Member

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    Essentially yes. Many of us chrono factory loads that we want to duplicate and use those baseline numbers for reference.
     
  5. Claymuncher

    Claymuncher Member

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    If my reloads are more than +/- 20fps I figure something is wrong with the load. I use 20 because that is what most factory loads will show on mine, usually tighter but a few get worse than that . I would think 50fps off is a lot and would not use that load, down range range who knows. My present hdcp load stays +/- 12 fps.

    I cannot imagine 50fps off between first pellets and center mass.

    Oehler 35P with dedicated light source.

    CM
     
  6. Johnny

    Johnny Well-Known Member

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    Jack, the rocket scientists are out to lunch, I just answer the phone. It's true the leading pellets will be faster. I think it's because the mass of the cluster hitting the choke results in pressure which forces the leading pellets out faster than the trailing pellets. Factories use a cylinder choke for chronograph measurements. As for where the 50fps " comes into play" , from the beginning to end of flight path. Don't know if 50fps is a valid number or not, just be aware that the leading pellets will be slightly faster.
     
  7. zzt

    zzt Well-Known Member

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    Jack, there are a few variables to take into account when using a hobby chronograph. The first is lighting. If you are using the sun as your light source, your chrono must be positioned in the shade so that no direct sunlight enters the view ports.

    Second, the ports must have a unobstructed view of the sky, and good diffusers should be used.

    Third, the distance from the muzzle to the first screen should always be the same. I find 8' to be the best with my chrono.

    Fourth, the path of the shot string should always be the same height above the screens and exactly down the centerline of the chrono.

    Fifth: the same choke should always be used. I found factory loads to always be about 50fps fast when I first started. Then I was not paying so much attention to details and I was using my fixed full chock barrel to do the shooting. I have subsequently found that using a more open choke not only decreases the average "gain" in velocity, it decreases SDs because there is less overall variation in the velocity spreads.

    When I chrono my 20b using a cylinder choke, I find the velocity "gain" of my light operated chrono to be between 20 and 25fps when shooting factory ammo. So although I don't have any way of knowing for sure, I take it from my results that choke constriction can add up to 25fps to results from a cylinder bore.

    BTW, Bob Dodd, a frequent contributor on this forum, was the one whose information and tips helped me get the best results from my chrono.
     
  8. willing

    willing Member

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    When I do a chrono session I fire 5 shots with a good(?)factory load, take average then find difference from stated value. Subtract/add the difference value from my reloads. It has usually been about 5%. YMMV.

    Bill
     
  9. Claymuncher

    Claymuncher Member

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    zzt : Oehler recommends 3-4ft. from entrance and advises using the most open choke available. Different settings for different chrono's?

    They also say my Oehler 35P typically reads 2-5% higher than factory velocities.

    CM
     
  10. whiz white

    whiz white Strong Supporter of Trapshooting Banned

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    I used to teach mathematics and physics... you're splitting hairs here.

    It is all relative. If a chrono catches the first BB in a shot string, then it also catches that first BB in the second "screen." I have to assume if probably "catches" a mass of BB's large enough to cause a reading... maybe it's center mass, but it is not relevant. I would be more concerned with what the chrono standard deviation yields.

    Do this: Chrono brand new factory shells, keep data, and build your reload from there. It really is immaterial which part of the shot string the chrono reads, it is only important that each screen reads consistently and similarly.

    H-S Precision here (gun manufacturer; sporting and tactical) has a full room of dedicated Oehler equipment for chronographing their test barrels they make for Federal, Remington, et. al. I've used them, and my "home" chrono yields similar results. For us hobbyists, the Chrono's available to us are just fine for checking relative reloads.

    Some of the above posters elude to this.

    IMHO.

    WW
     
  11. mtjack

    mtjack TS Member

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    Thanks all for the information, especially about distance, and choke.

    Originaly my question came up because I had developed a load (AA hulls, downrange DRXL 1oz wads, Fiochi 616 and 16.4 gr. Claydot powder) which was one of several provided by Alliant Powder when I emailed them for recipes using Claydot and Fio 616 primers. Shooting these through an IM choke approx 10 feet from chrono resulted in an avg speed of 1163fps, or pretty much what Alliant indicated. Since my AA hulls were worn out after many reloads, I replaced them with Rem STS( because that was what I could get at the time) Substituting the Rem hulls for the AA and increasing the powder charge as per a Alliant developed load resulted in an increase in speed of almost 100fps. Not what I was looking for. I've since started using a Downrange DRRT12 wad with the same powder chage am am getting the same velocity as my original load, I thought.

    After reading the above, I have learned that I may need to reshoot them as I had crono'd them at different times using an extra full choke on my unsingle and a mod and IM on my over/under most likely resulting in the large ranges. (when I say range, I'm talking about low to high)

    I may also crono several known factory loads to get a better picture of what my crono is seeing.

    Thanks again,

    Jack
     
  12. halfmile

    halfmile Well-Known Member

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    Deducting 30 has been my benchmark, years ago measured Federal paper factory loads and our reloads. the results seemed to bear out the 30 FPS deduct.

    HM
     
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