Chronograph accuracy

Discussion in 'Equipment Discussions' started by Razor18, Oct 27, 2010.

  1. Razor18

    Razor18 Active Member

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    Hi Guys,

    I have a ProChrono Digital chronograph, and am very satisfied with it. On the weekend we put three different types of chronographs behind each other. The difference between max and min readings were sometimes as high as 100 fps! How can I be sure my readings are correct from now on, in other words how can you verify your chronograph is really accurate?
     
  2. trebark

    trebark Well-Known Member

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    An age old question!

    This summer I ran the same experiment with my Chrony Master. Put a ProChrono behind it and the two were generally within 5fps of each other with my Chrony Master generally being 'faster'. Several weeks later, ran the same experiment with a CED. Again, the Chrony Master was faster, but this time the differential was 30fps.

    Not sure exactly how to verify the accuracy.
     
  3. kcebcj

    kcebcj Well-Known Member

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    I have a Ced M2 and shoot through it all the time. At first it bugged the hell out of me wondering if it was accurate. I have concluded its close enough as the dial-ups Exbal spits out using the velocities is close. I have to verify and tweak things anyway so I just go with it.

    I have no idea how one could verify the accuracy one to another.
     
  4. Bearwalk

    Bearwalk Active Member

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    I run a CED M2 in line with a Pro Chrono. I generally get 0 to 5 fps differences. Both return single digit std deviations. After years of frustration and trial and error, here's how I finally get repeatable and dependable results:


    • Pro Chrono is set at 18' and CED at 21' to avoid muzzle blast. Use a ballistics program such as Ballistics FTE or Loadbase to correct your downrange velocities back to the muzzle. Adding 10 to 14 fps usually does it.
    • The CED M2 has infra-red screens with foam board "coffin" to ensure even lighting. Coffin is painted flat black.
    • Pro Chrono is shaded from direct sunlight with plywood on top of our overshot protection. For me, direct sunlight has always resulted in bad data. I avoid it like the plague.
    • I use surveyor tripods to get as stable a platform as possible. I have found that camera tripods move around from the blast and wind too much. Any minor movements will ruin your results.
    • For those with CED's, don't forget to tighten your sensor mounts with the provided pressure screws. Don't ask me why I put this reminder in.
    • I level the chronos in both directions and ensure my bullets are passing as as level and as square as possible thru the screens. Angled shots can produce 6-12 fps errors.
    • All surfaces are painted flat black or gray to avoid reflections. This includes all tripod and chrono surfaces that the bullet may "see".
    • I shoot on sandy soil, so I use a brown tarp underneath my setup to avoid ground reflections.
    • I never shoot over snow. Never had any good come of it.
    • I put in new batteries every other trip to ensure stable power.
    Only thru this level of "anal-ness" have I been able to obtain useful shot data. I can't begin to tell you how much reliable velocity data has improved my load development. Coupled with now-useful models in Quickload, my load development and has become a quick and enjoyable process.
     
    Last edited: Oct 30, 2010
  5. cowboy

    cowboy Well-Known Member

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  6. Bearwalk

    Bearwalk Active Member

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    I can grab a pic next time I'm up there. Basically, we have a heavy structure of horizontal beams fill with pea gravel over our 200 yd range. They are about 9' off the ground. They start out close together and then progressively get further apart. The idea is that the shooter can never see daylight with his muzzle, thus a bullet can't escape the range from an AD. At 20' feet out the beams are about 3' apart. I just throw some scrap plywood we have laying around on top of the structure to block direct sun. They great thing is I still get plenty of ambient light, just not direct sun.

    On my CED M2, it doesn't matter since I use infra-red screens inside a foam box, or "coffin".

    Side note: By using two chronos, I pretty much know if they're lyin' to me.


    I hear what you're sayin' about no confidence in chronos. I had actually given up on them for a couple of years since I could never seem to get repeatable results that made sense. Finally, I started doing a lot of research on factors that can cause trouble with chronos. My "list" I posted above is the results of that research and a lot of trial and error.

    I hate to sound repetitive, but once I could start relying on my chrono results, it was a "life changing" experience. LOL. Not only did I shorten my development process, but my handloading improved and so did my accuracy. You see, once I started believing the chrono, I quit blaming it for the crazy spreads I would sometimes get. Instead, I started examining how I could improve my handloading to produce steadier velocities. Now, the whole process is working the way it should with good test data now feeding into improving "manufacturing" which in turn results in better performance at the target.

    Now, if I could just improve my trigger control....
     
  7. phorwath

    phorwath Well-Known Member

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  8. Hntbambi

    Hntbambi Well-Known Member

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    Get an Oehler of you have any doubts on the results of a chronograph. The 35P has a proof channel and it compares channel one and channel two for each shot. If one is out of whack, it throws that reading out.

    If you have multiple chronographs set up back to back, you will see a reduction in velocity on the farthest chrono. The bullet is constantly slowing down from the time it exits the muzzle, so the numbers will be lower farther out.
     
  9. Bearwalk

    Bearwalk Active Member

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    Would love to have an Oehler. They're awesome. Cost 3 times as much as CED M2, though.

    You should always use an external ballistics model to correct your velocities back to the muzzle. Otherwise you never truly have a muzzle velocity and variances from session to session could be significantly influenced by set-up distance.
     
  10. phorwath

    phorwath Well-Known Member

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    True, if you set them up in series, with the start and stop skyscreens from each chronograph set up one following the other. As in start followed by the stop skyscreen for the first unit. Then start followed by the stop skyscreen for the second unit, etc... The better way to set up multiple chronograph skyscreens is as illustrated in my Thread:
    http://www.longrangehunting.com/forums/f19/new-oehler-35p-new-skyscreen-rail-first-test-data-60778/

    A fellow would want to set up duplicate or triplicate chronographs' skyscreens such that the bullets fly over ALL of the initial 'start' skyscreens firstly, and then over ALL of the second 'stop' skyscreens secondly. With three individual chronographs, and three different sets of skyscreens, the bullet would fly over the three 'start' skyscreens first, and then over the three 'stop' skyscreens secondly. Set up in this manner, the speed of the bullets as fired over the multiple chronograph units will, in fact, be virtually identical. And if all the chronographs were perfectly accurate and precise, each chronograph would record the same velocity for each bullet fired.
     
    Last edited: Nov 5, 2010
  11. Varmint Hunter

    Varmint Hunter Well-Known Member

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    As a test, I placed my buddy's Chrony directly behind my Oehler 35P which was about 10' from the muzzle. The Chrony read aprox. 100 ft/sec faster with nearly every shot. I packed up the Oehler and shipped it back to have it checked out. According to Oehler ran a few tests, it was recording velocity well within design limitations. Presumably, the Chronoy was not.

    If two chronographs read aprox the same velocity I think it is safe to assume that they are both accurate. However, if they don't read the same you are kind of at a loss to know if it is caused by one, the other, or both.
     
  12. phorwath

    phorwath Well-Known Member

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    If you compared the Oehler 35P to a Shooting Chrony and there was 100 fps difference between the two, I would have told you the Shooting Chrony was the unit in error. The Shooting Chrony is a Cracker Jacks gift version of a chronograph compared to the Oehler 35P.
     
  13. Code4

    Code4 Well-Known Member

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    A chronograph can only ever be a rough guide. You have to shoot for accuracy and over the distances you plan to use your rifle on to get a correct idea of what is happening to your projectile in flight.