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Thread: Chronograph Errors!

  1. #1

    Chronograph Errors!

    I had a cracking day out shooting with WMS in Wales yesterday; very good day and very informative (if a little wet). But not without its shocks...!

    I was using my 6x47 lapua and throwing 105gr rounds down range at a chrono'd 2855fps. Using 2 ballistics programs that Andrew (the WMS chap) and I had we could not calculate the elevation and windage for any ranges beyond the zero distance with the figures I had speed and BC. Through a series of calculations and tests at ranges from 100yds to 644yds we finally calculated that the rifle was throwing out rounds at around 3125fps! That is a serious chronograph error of 270fps!!!

    Has anyone else had a similar issue with inaccuracy of their chrono that they are aware of?

    Al

  2. #2
    I would not be amazed with 11.5% error. I think people overestimate the accuracy of these instruments.

    Did you check against another chronograph?

    Are they ever calibrated once they leave the factory? Are they even checked there?

    What accuracy do they claim? What if the batteries are going flat, does that alter anything? What if it has sustained slight damage or a minor twist?

    So many factors that could cause a greater error. It's all ballpark.

    Section 161 of the Highways Act 1980 (England & Wales) makes it an offence to discharge a firearm within 50 ft of the centre of a highway with vehicular rights without lawful authority or excuse, if as a result a user of the highway is injured, interrupted or endangered.

  3. #3
    The rounds I had were my standard loads that have always been velocity accurate through my chrono +/- 10fps. I didn't have the chrono with me as I thought I knew what the load was doing. Really glad we took the time to work through the errors. Just amazed at how cr4p my chrono is!

    Al

  4. #4
    From memory both Chrony and ProChrono claim 99.5% accuracy. I've no idea what their QC is like, though.

    I suppose if there was some other possible explanation for the discrepacy between the recorded velocity and the observed ballistics, the WMS folk would have sussed it out.

    Pedantry corner:
    The round goes into the chamber, but ceases to exist once the primer has ignited.
    The bullet then makes its way to the target.

  5. #5

  6. #6
    Quote Originally Posted by Dalua View Post
    From memory both Chrony and ProChrono claim 99.5% accuracy. I've no idea what their QC is like, though.

    I suppose if there was some other possible explanation for the discrepacy between the recorded velocity and the observed ballistics, the WMS folk would have sussed it out.

    Pedantry corner:
    The round goes into the chamber, but ceases to exist once the primer has ignited.
    The bullet then makes its way to the target.
    The chrono I have is a ProChrono. I think that I may do some more testing to try to establish the actual error before complaining.
    I will ignore the Pedantic commentary as the point of communication is to be clearly understood [Ref Steven Fry 2012]

  7. #7
    I had the same fault some time ago with my prochrono some of the ds2 lads were borrowing it , i was about to shoot it as a bad deal but it was to wet to get to there location i told them i would pick it up as soon as pos ,but by the time i got back to them it had stopped raining and dry'd out the chrono worked fine with no errors coming back as before ,i think it just did't like getting rained on bit like all of us! Now reads ok and compares to the data on the target as per the computer readouts within 1/2 inch. 2p worth try doing a ladder shooting at said ranges see if they match



    Quote Originally Posted by 6x47 View Post
    The chrono I have is a ProChrono. I think that I may do some more testing to try to establish the actual error before complaining.
    I will ignore the Pedantic commentary as the point of communication is to be clearly understood [Ref Steven Fry 2012]

  8. #8
    I'd expect the spread to be pretty low as these chronographs are timed with a simple count down clock and crystal.

    The pdf download says "+/-1% of measured velocity, or better" so I'd call that about 30fps either way.

    Have you run some fair grade 22LR over it? They should give you pretty low (10fps) spread and something like Winchester subsonic is close to the 1055fps most of the time.

    When you chronographed the loads were they within, say, 50fps spread? Just wondering if the unit is behaving or having an off day. My current Chrony Master has sulks under some lighting conditions but is generally very precise and consistent (as my loading).

    I had a discussion with a colleague a few years back an he insisted on checking my data with his very expensive calibrated unit: the CED I had at the time was within 2fps and just as consistent.

    Best of luck as they're fussy creatures but useful sometimes.
    Last edited by Barkingsnake; 17-01-2014 at 19:40.

  9. #9
    I will give my unit a go with my 22LR, thanks for the recommendation.

    In terms of lighting, what are the best conditions? Overcast, sunny etc. Would it be worth putting a cover over top of the beams?

    Al


    Quote Originally Posted by Barkingsnake View Post
    I'd expect the spread to be pretty low as these chronographs are timed with a simple count down clock and crystal.

    Have you run some fair grade 22LR over it? They should give you pretty low (10fps) spread and something like Winchester subsonic is close to the 1055fps most of the time.

    When you chronographed the loads were they within, say, 50fps spread? Just wondering if the unit is behaving or having an off day. My current Chrony Master has sulks under some lighting conditions but is generally very precise and consistent (as my loading).

    I had a discussion with a colleague a few years back an he insisted on checking my data with his very expensive calibrated unit: the CED I had at the time was within 2fps and just as consistent.

    Best of luck as they're fussy creatures but useful sometimes.

  10. #10
    British lighting? What a laugh! I live in the far North and so this time of year anything between 1000 and 1400 provided it's not raining or snowing!

    The shields that go on top of the unit are needed when you have blue sky or broken cloud: the real sky colour is pretty "black" and so the plastic bits give the plastic bits give the photosensors something to look against.
    If you have moderate overcast it'll work well without them. If it's very thin cloud then they might as well be left on.

    I've just done a quick look at the spec data at Pro Chrono Digital
    and if they're using a 1.3MHz crystal then even the most crappy grade will barely give a 1 or 2 fps error if my sums are right.

    This make sense?

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