standard deviation and accuracy

Keeping it simple, SD is a measure of uniformity used by engineers to measure quality of ammunition, as well as all manufactured product. At longrange, ammunition with high SD will show variations in elevaton and scores will suffer. This is not debatable and a simple ballistic program can show this. Professional engineering evaluaters have SD standards which must be met.

Regfareds,

Peter
 
This is the exact notion I was alluding to in my prior thread about chronograph accuracy. If your individual shot readings are +/- 0.25% accurate AT BEST (ie +/-7fps on the basis of 3000 fps); then the resulting SD of 5 that the computer calculates is complete garbage! I maintain the stance that the SD and ES outputs on chronographs readily available to the general public (ie <$500) are for entertainment purposes only. The best function the chrony can provide is a ballpark +/- 20fps velocity for use in ballistic calculators. After that, it's pure folly. Just looking at the facts for reality, despite what we'd all like to believe is possible through $500 technology.
 
I think that the right mentality here is needed- chronographs are useful in certain ways, but it is important to understand what they can and can't do. the PVM appears to have some excellent software features in it, and may provide better accuracy than lesser models. But... The overarching issue usually stems from the data collection side (timing), not the computer side (SD calculation). The PVM quotes an error of <1% from displayed value. Before going into the numbers of what 1% is, what is the 1% OF? 1% of the velocity reading? 1% of the calculated statistics? If they claim accuracy of 1% for the velocity readings, then it is mathematically unpossible to have statistics with a corresponding error of 1%. It would be nice if the manufacturers would release the accurate information to begin with, especially if they expect professionals to use their hardware.

The only information I found from PVM's side with respect to information was in the disclaimer at the end of the flyer:

1. Online-contents
The author reserves the right not to be responsible for the topicality,
correctness, completeness or quality of the information provided. Liability claims regarding damage caused by the use of any information
provided, including any kind of information which is incomplete or
incorrect, will therefore be rejected.
All offers are not-binding and without obligation. Parts of the pages
or the complete publication including all offers and information might
be extended, changed or partly or completely deleted by the author
without separate announcement.

Cirrus had professional engineers design up an airplane that was supposed to be exceptionally safe for the John Q Private pilot to transport himself around in. During flight testing, it was found the initial design needed bandaids of stall strips and a ballistic parachute because it would not recover from a spin before running out of reasonable altitude, if one was inadvertently entered. Not to pull the rug out from anyone's reality, but I've learned to perceive the word 'professional' with the utmost caution.

Again, the only way to accurately use the chrono's available today is to understand exactly what you can and can't ask of them.
 
I never really thought about it that way, the load I have with retumbo that was shooting 30 fps different on the Chrony is accurate, I shot it at 600yards at the range a couple weeks ago, the results were excellent. Vertical spread was negligible to my surprise! The 3 shiny dots at the bottom were the last 3 and the 2 at the middle right were my first shots (I moved so I could see where I was hitting easier)
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Off hand anyone ever re-calibrate your measuring gear? Measuring gear, whether scales or chronographes do lose accuracy over time. Not sure how much they lose over time or if it's possible even to re-calibrate them but ya might want to keep that in your head.
 
mulemania said:
I never really thought about it that way, the load I have with retumbo that was shooting 30 fps different on the Chrony is accurate, I shot it at 600yards at the range a couple weeks ago, the results were excellent. Vertical spread was negligible to my surprise! The 3 shiny dots at the bottom were the last 3 and the 2 at the middle right were my first shots (I moved so I could see where I was hitting easier)
image.jpg
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Click to expand...

If the Chrony has a rated 0.5% error rate, and I do believe that is fair, +/-15fps is within the design parameters of the device.

Seeing 30fps difference in numbers on a 3000fps muzzle velocity means all those numbers are the "same" as defined by the device.

ie 2985 and 3000 and 3015 IS the same number as it relates to error.

Many forget that all measuring devices have some level of error so the output value has to be taken with that context.

The correct way of calculating SD's and ES's with these devices is to include the error range with EVERY value instead of using the output value as an absolute. Very quickly you will find that the values including error overlap to the point where the math becomes pointless.

There is a bunch of math/stat mumbo jumbo about degrees of certainty, etc but the only thing that matters in this game is holes in paper.

If bullets land with 1" vertical at 600yds, I am going to call that load smoking awesome and couldn't care less what an LED readout tells me... except that it gave me enough info to dial a scope to hit paper.

One last point just to give something to chew on over coffee.....

Let's assume, we have some super duber accurate Olympic quality atom clock chronie that can measure accurately to the +/- 2fps. We tune out loads and we get output speeds to within a TRUE single digit spread.

Will that chronie account for changes in aerodynamic drag of the bullet due to variations in the bullet shape? Or Twist Rate?

Holes in paper, tends to simply the math alot....

Jerry
 
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