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determining chronograph accuracy?

Discussion in 'Shooting Related Threads' started by skeet_man, Jul 4, 2010.

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  1. skeet_man

    skeet_man Well-Known Member

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    Ran some loads over the chrono today. Shells are fairly consistent, but how does one know if the chronograph is giving you the correct reading? Would taking factory loads of a known velocity and shooting them over the chrono, recording the difference (if any) from the stated velocity, and read velocity, then adding or subtracting that difference from your reloads be an acceptable practice? Near as I can tell, there is no real way to calibrate a chronograph, or even to test its accuracy.
     
  2. BigM-Perazzi

    BigM-Perazzi Well-Known Member

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    My way of thinking is. Even though you may not get the correct velocity with the factory shells, you will get a baseline to compare YOUR reloads to...
     
  3. Paladin

    Paladin Well-Known Member

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    What DB said.

    Mine was hit by 1/2 of the sabot sleeve of a 12 ga sabot slug, of all things.
     
  4. BigM-Perazzi

    BigM-Perazzi Well-Known Member

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    That's what that piece of 1/4 clear lexan glued to the front is for!! ;-)
     
  5. Paladin

    Paladin Well-Known Member

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    If your unit is one piece, yes. The sabot sleeve piece took out the first light sensor body.
     
  6. Quack Shot

    Quack Shot Active Member

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    If your results are near the expected velocities and consistent enough to be believeable, then you are probably on the right track. Induction screens are the best with shotshells, but the light screens used in the inexpensive units will usually give consistent readings, good enough for the average reloader. I use my chronographs to test loads under certain conditions. Cold weather and hot weather performance can be one useful result. Consistency of a particular load can be a useful result. I've seen some combinations of components that did not perform as expected. The actual velocities and what your chronograph say might not be quite the same, but for most purposes, they are good enough. Neil Winston had posted a link to some good information about chrongraphing shotshell ammunition. I believe he advised using two units to help check the results. There is no factory shell that I know of, that would give you "exact" velocity results every time. There may be some "standard" shell for proof or pressure testing, but it's not to be found in a box of factory trap loads.

    "Proof" channel capable units are another possibility if accuracy concerns you. I have used two units back to back and I have found that they give a consistent and believable reading if I do my part. You could even put three units in line if you wanted to. I have one unit that seems to work fairly well for most purposes. If I need to double check my results, using two units together is my usual solution, although I don't get too many erroneous readings in the first place.

    I would look for Neil's link and read the info he has posted there. It's pretty much on target and gives some good advice.
     
  7. JACK

    JACK Well-Known Member Supporting Vendor

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    A bit off topic, but I find chronographing my reloads to be a tremendous confidence booster. $120 ProChrono

    [​IMG]
     
  8. phirel

    phirel TS Member

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    If you put two chronographs end to end and shoot one shell and get close to the the same readings on both, it is likely that both chronographs are correct. If you get very different readings, you know at least one of the two chronographs is wrong. One should expect some variation in velocity between shells. Very careful reloading (weighing powder/shot) will reduce some of this variation. It is also critical to use the chronograph in the same manner for each shot.

    Factories use a different type of chronograph (induction system) and that will give lower velocity readings than our light chronographs. Factories also publish nice round numbers for the velocities of there shells. They "analyze" there data using reconciliation and that is very different from calculating an average.

    Pat Ireland
     
  9. Don Steele

    Don Steele Well-Known Member

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    From what I read about the Oehler unit, the extra screens (may) be there to provide useful information on PRECISION...Not accuracy.
    I use a Chrono. to compare my loads to each other(monitoring PRECISION)....using published data.
    There are so many variables involved in acquiring Mv data from a chrono that it's not reasonable to expect to duplicate with a high degree of confidence any published Mv data. You'll get close...."in the ballpark". Even a "standard load"...produced under a rigid Q.A. system, then sent around the country to be used for "home comparison" can't model the exact conditions of the original test shots and that's where comparisons for Mv ACCURACY(not precision) begin to breakdown.
     
  10. willing

    willing Member

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    I shoot 5 factory shells, compute an average fps, compute difference from stated value. I then shoot 5 of my reloads and correct the speed by the diffence found with the factory loads. Is there a better way??

    Bill
     
  11. RunGunIPSC

    RunGunIPSC TS Member

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    "willing"(Bill) has got it right. If we go to a shoot & want a certain Dram Eq. for speed,we accept these without question. So use these to measure your reloads. Shoot 3 new shells over your chrono that is closest to what you are trying to develop,then shoot 5 of yours,then 2-3 more new shells. Yes it is costly in new shells but soon you will have a feel for what looks right or not. I use an old Ohler Model 11 for shotshell.
     
  12. halfmile

    halfmile Well-Known Member

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    The modestly priced (though adequate for shotgunners) chronographs work fine. Just add 30 FPS to your readings.

    We have discussed this before here.

    HM
     
  13. BDodd

    BDodd TS Member

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    I have and have used the Oehler 35P for many years; originally bought to monitor pistol velocities for competition. Oehler states right in the instructions for the 35P that shot shells are the least accurate over a homeowner's chrono - they suggest reducing readings by anywhere from 3 to 5% if my memory hasn't totally escaped. The final opinion with many years of trying to ensure accuracy of results with shot shells was to use the same gun always, same barrel always, most open choke possible, try to have good weather, and use a factory shell as my base line to either be matched or to exceed or to be less than that base line. I pretty much ignored the numbers provided and just used the factory shells for my "target" velocity. During that time, I did determine that my 28" barrels on a M.32 Kgun with skeet tubes seemed to produce as close as any other combination to what is printed on factory boxes and that became my test gun. .....breakemall.....Bob Dodd
     
  14. W.P.T.

    W.P.T. TS Member

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    I look at the flap thats on the boxes when I open them and figure they are going to be close enough to that number being as I trust that the factory probably knows what they are doing ... If anyone ever challenges my shells they are challenging Remington, Federal, Rio, or Winchester in that order ... Thats what makes shooting new shells so nice, as long as they go boom and the target breaks is all that matters ... If there was an advantage to reloading all of the Top Guns would be shooting reloads, that you can bet on ... WPT ... (YAC) ...
     
  15. Neil Winston

    Neil Winston Well-Known Member

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    Yes, WPT.

    Neil
     
  16. phirel

    phirel TS Member

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    WPT- My experience with one of the factory shells you mentioned indicated that the actual velocity of the shells was not related to the velocity stamped on the box.

    Pat Ireland
     
  17. EE

    EE Banned User Banned

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    And that would be Rio.

    EE
     
  18. BunkerGuy

    BunkerGuy TS Member

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    Since the chronograph will turn different values depending on set up, including distance, I test fire factory Remington or Winchester STS or AA 2 3/4 dram 1 1/8 oz shells at the beginning of each session for a reference. Over the years, I've found this factory velocity is the most consistent -- unlike the "3 dram".

    If the Oheler 35P says the velocity is 1145'/s +/- 5 feet or so, I know I've got the setup right. . . This makes session to session results consistent. Except for temperatures.

    - - Bill
     
  19. W.P.T.

    W.P.T. TS Member

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    Pat,

    By my not checking the shells I figure they are close enough for piece work and thats one less thing I have to worry about ... What difference do you think it makes if a shell Chrono's at 1220 fps or 1290 really makes at the point of impact ..? I say not enough to matter ... WPT ... (YAC) ...
     
  20. AveragEd

    AveragEd Well-Known Member

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    The makers of the two chronographs I use advised me to DEDUCT 30 to 35 fps from the readings when clocking shotshells as the leading pellet is clocked, not the mass.

    I had something interesting happen yesterday that's kind of related to this topic. I'm doing a gun review on the new Remington 870 SuperSlug and wanted to shoot three shells each of eight different saboted slug loads over the chronograph while shooting 100-yard groups. Neither chronograph would register a speed and would either display an error for an object crossing the "start" photocell but not the "stop" photocell or vice-versa.

    Before going to the range, I had cut open one of each load to photograph the bullets and sabots. In found that they all had at least one "wad-like" object under the sabot - most had two, one fiber and one plastic or two plastic. I can only assume all those objects passing over the photocells confused the units. Our son and I even erected a 10'x10' canopy over the tripod in case the bright sunlight was the culprit.

    I'm waiting to hear back from the chronograph manufacturers but that's my theory on the cause. Both chronographs worked fine while testing loads in two centerfire rifles while the SuperSlug's barrel was cooling.

    Ed
     
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