Testing E Target Accuracy

Discussion in 'Competition Forum (All Calibers)' started by Bindi2, Apr 11, 2019.

  1. Bindi2

    Bindi2

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    Buy a roll of opaque cooking paper or grease proof clear. Make a square to cover the aiming mark divide in to quarters with a pen line. Place a new aiming mark on the ET at a medium to long range. Fire three rounds into each quadrant of the aiming mark with just two of the total number as close to the centre as possible. Record the X Y coordinates for each shot if the log can not give them to you. From the record mark where each shot is on the grease proof cooking paper place this over the aiming mark for best fit.
    Things that you will notice.
    X does not mark the centre of the target. The acoustic centre is mobile as the day goes on and more tests are done The centre area could be up to 30mm wide. You will see this in time with more tests done. The size or mobility does not affect accuracy.
    The wider the shot is out from the centre greater than half way out you will notice the error is almost always in the shooters favour. This is also where you see if the mics are true. A wide error indicates there is a problem in closed systems. A wide error or constant error on an open system is the doppler effect when the mics are correctly placed.
    A good closed system will record inside less than half the diameter of the bullet some even better.
    It is all in the construction of the frame, timber used and age of. Frames do warp under tension, which is why we have moved away from tension to solid sound chambers.
    The first shots fired on a brand new target should be a test which is kept as the base line info for that target.
     
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  2. steve_podleski

    steve_podleski

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    I use 2mm graph paper and put about 30 or more shots evenly spread on the the 6x6 target.

    What is the "doppler effect"?
     
  3. Bindi2

    Bindi2

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    The doppler effect is what the wind does to the sound waves. There is a explanation in a thread linked to the Hexta site.
    Please explain how you do this test . How do you see actual and recorded.
     
  4. Crow

    Crow Silver $$ Contributor

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  5. sdean

    sdean Gold $$ Contributor

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    I don't believe any of the old testing results included the newer 8 sensor open targets. Why don't y'all wait and see what gets certified by the NRA. That is what will count whether you like it or not.
     
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  6. GeoffR

    GeoffR

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    It's not really Doppler as we are not measuring a frequency - but it is similar. Doppler is basically the change in frequency as seen by a receiver away from the transmission frequency of the source - a function of the relative velocity of one against the other. The classic example used to illustrate this effect is the ubiquitous sound of a train whistle as it's approaching versus its sound when departing (higher pitch when approaching, lower pitch when leaving).

    A bullet emitting a shockwave is not transmitting a frequency as such so we are not measuring this. We are measuring the time it arrives at a sensor - it is simply a shock on the sensor. The sensors have a resonant frequency and will ring a bit, but we are not measuring that either - and that frequency has nothing to do with this problem.

    But what is affecting the shockwave with a moving air mass (like wind) is similar to doppler (as I said above). It is the effect of a slightly different speed of sound relative to the sensors and bullet in the direction of [sound] travel, and those sensors the sound is moving away from. We assume that the speed of sound is constant across the entire target face - we have no choice in this.

    The error is roughly 1/2" per 10 mph. It effectively changes the acoustic centre of the target. It is especially problematic in a fishtailing wind. The distance traveled by the bullet to the target is inconsequential. So a 5mph wind can potentially push accuracy outside of the proposed (or is it set) 1/4" (6.25mm) accuracy mandated by your NRA.

    The error cannot be detected/calculated by mathematical means because the air mass is moving across the entire face: all the sensor times line up (so to speak). Any other errors can be detected and accumulatively quantified however (but that is a different topic). But the effect of wind is relatively easy to prove with some basic maths.

    Errors induced due to shockwave distortion resulting from a non-perpendicular trajectory can be detected and measured - as are other errors from other sources.

    A constant crosswind will probably be noticed by a shooter and accommodated as normal by a sight adjustment - that may be a bit coarser than expected. But a fishtail will have a shooter chasing it from side to side with accentuated adjustments in an effort to keep up.

    Convection (mirage conditions at the target) can introduce vertical component also.

    Due to the constantly changing acoustic centre relative to the fixed visual centre - that the shooter is aiming at - an impact position calculation will be inaccurate by an amount up to what I said above. In other words, accuracy is compromised and there is nothing that can be done about it with an open target. It affects any open target - it doesn't matter who makes it. It certainly affected mine when I was playing around with all this.

    You don't see this effect in a closed target as the air inside the chamber is still. Or should be... Having said that, closed targets have a different set of problems. But most if not all of these can be dealt with.

    Having said that, in still air an open target can be extremely accurate. Unfortunately, completely still air rarely prevails.

    Now I guess I have to wait for more blowback for saying things some of you don't want to hear.

    Geoff.
     
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  7. GeoffR

    GeoffR

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    8 sensors won't fix this problem.
     
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  8. adamjmac

    adamjmac

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    “The error cannot be detected/calculated by mathematical means because the air mass is moving across the entire face: all the sensor times line up (so to speak). “

    not exactly...
     
  9. Bindi2

    Bindi2

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    Thankyou for that explanation GeoffR much better than I could do. Only 1/2" per 10mph Mmm I need to recalculate the wind speeds here then or look at more site testing for wind. I know flags lie and there are no flags at the target face. Wind mills are banned. I actually wanted Steve Podleski to explain his method.
     
  10. spclark

    spclark Gold $$ Contributor

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    Please explain.

    I would expect GeoffR’s comment to hold true if air movement across a sensor arrayed target were consistent at the moment a shockwave registers.

    Add air turbulence to the scenario (in open systems or closed once the membranes’ integrity has been compromised in use) how does one factor in the differences for what each sensor’s input shock experiences between moment of passage at array then at each individual sensor?
     
  11. Bindi2

    Bindi2

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    With a sealed sound chamber that is being compromised not yet out of service the groups start opening up in the recording of them though the actuals have not. It takes very little wind to cause this upset. One of our test targets we used a material that was used as self sealing fuel tanks in WW2 aircraft ( what I was told )in place of the rubber skins this gave a very high shot count without problems but the weight was just to high for what we needed. The cost factor was up there but the test was abandoned when the cassette idea surfaced. The wind and sound are strange bed fellows. When rubber under tension has a bullet pass through it the resulting hole added to all the others create small fingers that wave in the space causing havoc with the sound waves or a bulge that protrudes out past the line of sight between the mics having the same effect opening groups add wind into this and Houston we have a problem. The wind blowing across the target caused more grief than at the target.
     
  12. steve_podleski

    steve_podleski

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    I think this is a misunderstanding of the doppler effect.

    I measure the actual by using the grid from the vertical and horizontal axis whose origin is the centre of the target. THe recorded is given in the csv file generated by etarget system.
     
  13. steve_podleski

    steve_podleski

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    Not quite sure I understand your reasoning. The sensors are picking up the shockwave and not the sound wave. The shock wave is moving at a different speed than the sound wave. Wind will affect sound speed which will slightly affect Mach number (M=V/a) which will slightly affect shock strength and angle (as will bullet speed) and therefore the point of impact. And the bullet/shock will align with the relative wind. So I don't see how wind affects an external system with 8 sensors which is supposed to measure bullet trajectory angle into the target
     
    Last edited: Apr 12, 2019
  14. Bindi2

    Bindi2

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    Steve the centre of a ET is not X marks the spot. It is an area Which is why you need two sheets for plotting actual and recorded.
     
  15. steve_podleski

    steve_podleski

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    Not sure what you mean by two sheets. I coincide the virtual/acoustic and paper centers by removing the average error in x an y resulting in the random errors.
     
  16. GeoffR

    GeoffR

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    Steve,

    The [first] shockwave emanates from the bow of the bullet at an angle dependent on the velocity of the bullet. It's the only one we're interested in. The slower the bullet the wider the angle. It moves at the speed of sound perpendicular to that angle - NOT perpendicular to the line of travel (trajectory). The speed of sound is essentially a function of the density of the medium through which it is traveling (air in our case) and its temperature. Pressure has a very mild effect and need not be considered in this case

    Any moving air mass will affect all sensor planes concurrently. I simply don't see what having two sensor planes give you if they are being equally affected. Yes, you can probably determine a trajectory from two planes, but both offset by the same amount. Again, I don't see what that gives you other than an x,y offset. You still have a moving acoustic centre relative to the fixed visual centre.

    Where do you get this bit about the shock wave versus sound wave and their differences?

    We are only interested in the shockwave. There is plenty of material about explaining the properties of supersonic shockwaves.

    Geoff.
     
  17. steve_podleski

    steve_podleski

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    The velocity component normal to the shock wave is M*sin(theta) where theta is the shock angle and I doubt it coincides with the speed of sound.

    There is a lot of confusion of Mach angle with shock angle (check this website for the difference http://www.dept.aoe.vt.edu/~devenpor/aoe3114/calc.html under the conical shock section). Shock angle depends on Mach number AND cone/tip angle.

    The shock moves forward at the same speed as the bullet. But the important speed for the reception of the shock to the sensor is M*tan(theta) and that component of speed is normal to the bullet trajectory and that speed is also not the speed of sound.

    I don't understand the discussion of two sensor planes. What do you mean by 'two sensor planes'? If you mean the 8 sensor system, I think that they are used to determine speed (the SM and SMT people may explain this).
     
    Last edited: Apr 13, 2019
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  18. GSS

    GSS

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    Man I feel like a dope.
    You scientist and math wizzes are amazing.

    Sorry for the diversion from the highly impressive match / science educational posts for those that can make any sense out of them.

    I'm a simple Man,
    I put the e targets up, shoot at them and compare what I see on paper to what I see on the tablet.
    I especially like to go by the shots that are right on the line, just in or just out if I really want to get precise.


    So far I haven't seen anything out of line except one shot on a 600 yard target in 25 - 30 mph winds coming from 11 o clock that was probably moving the target and the carrier a good 3 - 4 inches forward and back.

    All 22 shots on that target were right on except one wide 8 up around 2:30 where a gust got me while I was on the gun. That shot was probably around .25" off towards 1pm on the tablet from where it was on paper.

    All shots from the previous two 22 shot strings with lesser wind conditions were all right on as far as human eyes could see on paper vs zooming in on the tablet image...

    Like the old days on paper targets when we simply looked at it and marked it what it was, in or out...

    ShotMarker, Excellent Product ( Huge Thanks Adam ) good enough for me...
    Also priced so that my club budget let me buy 10 of them...

    I have all my 2019 CMP matches and all my NRA Approved and Registered matches signed off on as approved from both organizations with me specifically stating in all match bulletins that ShotMarker Electronic targets will be used in those matches.

    I understand the Silver Mountain Solo customers feel the same sense of plenty good enough for them...
    Their NRA and CMP Matches are also getting Approved. Very good thing IMO...

    GREATLY... Looking forward to much practice and smooth running matches on my affordable / quality E targets in 2019 and beyond.

    Cheers All,
    George

    Edited to Add:
    I am not a paid salesman, inventor, developer or designer.
    I have no affiliation or nothing to gain no matter what E target anyone buys.
    I am VERY Grateful to Silver Mountain and ShotMarker for delivering a great product that individuals and clubs can afford.

    I would bet my last $100 that I am not alone in that appreciation to the two companies that delivered excellent, do all we need them to do affordable systems for us shooters in the US and to help us Match Directors GROW the SPORT...
     
    Last edited: Apr 13, 2019
  19. chkunz

    chkunz Gold $$ Contributor

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    The reference looks like it is for the case where the gas is moving over a stationary object (like an airfoil in a wind tunnel) rather than the case where the object is moving thru stationary gas (like a bullet thru air). It has been a long time since I worked with this sort of thing so I may be mistaken.
     
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  20. TUFFLUCK

    TUFFLUCK Silver $$ Contributor

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    Im curious what the error is at a 300 yard mid range match after 60 shots by a 1/4 moa Dasher on the same F Class repair center on a conventionally scored paper target match.

    Asking the NRA High Power Committee to establish certification standards, is very much like asking the FAA to certify UFOs. The requisite knowledge and competence to fully understand, comprehend, and then apply the science isn't held within the High Power Committee as a whole.

    A potential path to follow...

    1. Establish what the mean statistical error is of the current gold standard, a paper manually scored match. Obtain this data from either polling the high power/match directors of the high power community, or form a study group that obtains this data from the observation of actual matches. From there, an actual precision standard with a meaningful statistical mean can be determined. Paper matches are not able to meet the .25" standard 100% of the time, so a real world data derived # needs to be assigned to the current gold standard. First.

    2. NRA Partners with industry. Despite what the threads on here may indicate, there's less than a dozen minds in the world that have the relevance and competence necessary to apply acoustic target technology to NRA High Power disciplines. Leverage the industry leaders of this tech, to develop proposed certification standards based off the capability of the technology.

    3. Analyze the proposed certification pathways presented by industry, and compare with current paper match practices, to determine if the science behind the industry proposed certification pathways can meet/match/improve precision based on current paper match precision.

    4. Keep in mind the charter of NRA comp shooting, by remaining true/faithful to history, past records, and the intent/practices behind current disciplines, and leveraging new technology to improve the shooter experience and grow the sport.
     
    Last edited: Apr 13, 2019

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