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Es/sd

 
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  #1  
Old 03-16-2010, 12:00 PM
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Es/sd

Sorry for the newbie question but what is ES & SD? I see posting about it for load development but don't know what they are talking about. How does one test for them? Thanks for the info!
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  #2  
Old 03-16-2010, 12:10 PM
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Re: Es/sd

ES = extreme spread of muzzle velocity. The highest muzzle velocity minus the lowest muzzle velocity.

SD = standard deviation of a set of muzzle velocities. This describes the expected spread of muzzle velocity - above or below the true mean of muzzle velocity based on a set of recorded muzzle velocities. You could Google search the term and get the exact definition. It's used to analyze data sets in Statistics.

Muzzle velocities are typically obtained with chronographs, which typically cost somewhere between $125 and $400, depending on the Brand and Model.
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  #3  
Old 03-16-2010, 12:23 PM
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Re: Es/sd

ES = extreme spread = the spread between the highest and lowest velocity in a group.

SD = standard deviation.

A defintion of SD from wiki...

Quote:
shows how much variation there is from the "average" (mean). A low standard deviation indicates that the data points tend to be very close to the mean, whereas high standard deviation indicates that the data are spread out over a large range of values.

Standard deviation - Wikipedia, the free encyclopedia

These numbers are derived by shooting through a chrony that measures bullet velocity. I personally dont put much stock in them because chrony's are not precision instrumnets. I have seen a lot of inconsistancies with my chronys (I have two) When I shoot them back to back their velocites will vary anywhere from 5-60 fps.

If you shoot a 5 shot string and get the following velocities...

3056, 3067, 3094, 3094 and 3069, Your ES is 3094 - 3056 = 38

However, the odds are very lilkely that your actual ES is smaller.

The reason being, Your chrony is accurate to plus or minus "x" fps. The shot that measured 3056 might actually be 3070 and the shot that measured 3094 might actually be 3080. For this reason, I put almost no stock in chrony measured ES's. I use actual group results to determine my best loads.

A good experiment to try is go out and shoot a 5 shot string, record the results and repeat the next day under similar light conditions (light conditions will affect readings) I can pretty much guarantee you that you will get different results.
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  #4  
Old 03-16-2010, 01:28 PM
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Re: Es/sd

Quote:
Originally Posted by MontanaRifleman View Post
If you shoot a 5 shot string and get the following velocities...

3056, 3067, 3094, 3094 and 3069, Your ES is 3094 - 3056 = 38

However, the odds are very lilkely that your actual ES is smaller.

The reason being, Your chrony is accurate to plus or minus "x" fps. The shot that measured 3056 might actually be 3070 and the shot that measured 3094 might actually be 3080. For this reason, I put almost no stock in chrony measured ES's. I use actual group results to determine my best loads.
Mark,
So even when the recorded velocities are collected in the same lighting conditions within a 20 minute period of time, you place no confidence in the ES?

I could understand not trusting the accuracy of the numbers; for example, is it really 3057 fps or is the true velocity 3068 fps. But I don't understand the dismissal of the variance (or spread) of the velocities on a given shot string. When I run two chronys in tandem and for five shots, the difference between the recorded velocities on the two different chronys for each shot is 15, 15, 16, 18, 16 fps, then I conclude with confidence that I've closely established the spread between the highest and lowest recorded muzzle velocity for those 5 shots.

Particularly when I receive the same type of delta between recorded velocities day after day after day over the dual chronograph setup.

I believe there is validity to the conclusion that the accuracy of the recorded velocity can vary under different lighting conditions; that a bullet whose true muzzle velocity is 3000 fps might be recorded as 3020 fps in one lighting condition and 2980 fps under a different lighting condition. But when two different chronographs spit out muzzle velocity differences within a 4 fps window, shot after shot in the same lighting conditions, I have to reach a different conclusion regarding the reality and accuracy of the ES.

I place high confidence in the variance in velocities if my deltas over the two chronos are consistently within 4 fps. I accept the fact that the accuracy of the recorded mean velocity of a particular load from day to day under differing lighting conditions may be off a bit either high or low. The MV I input in my ballistics program is the mean of the means collected from separate data sets for that same load, with some consideration given to the different ambient temperatures for each set of data, since powder burn rates and muzzle velocity would be expected to vary under differing ambient temperatures.

Lastly, and only after I've selected my final load for a rifle, I like to collect velocity over the chronographs as far downrange as possible. Farthest I've collected the velocity data to date is 990 yds. Then I tweak the BC in my ballistic program until the predicted 990 yd velocity matches the 990 yd chrono'd velocity and see how the actual measured drops compare to the predicted drops. So far they match very well.

That's how I use and apply the data from my chronographs - for better or for worse.

Last edited by phorwath; 04-13-2010 at 03:55 PM.
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  #5  
Old 03-16-2010, 09:21 PM
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Re: Es/sd

Paul,

First I should mention that in my list of made up velocities, I didn't intend to type 3094 twice. No big deal.

If I was consistently getting differences of only 1-4 fps, then I would put a lot more trust in my chronies. But I am getting much bigger differences... 5-60 fps, set up back to back (and set up carefully). There is no way that I can come to any solid conclusion about the ES that these chronys are telling me. One might give me an ES of 10 and the other an ES of 60 with the same shot string.

For me, the bottom line is the actual results downrange that count.
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  #6  
Old 03-16-2010, 09:26 PM
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Re: Es/sd

10-4. With those MV inconsistencies, I understand.

Down-range groups and drops are the final say-so for sure. I use my chrono data as an aid to getting there - sometimes with less time and effort than without. I also like the down-range velocity confirmation. Combined with good correlation to drops, it helps ensure the ballistic software inputs and program are providing realistic predicted data at other ranges and under other atmospheric conditions.
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  #7  
Old 03-17-2010, 12:42 AM
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Re: Es/sd

Quote:
Originally Posted by phorwath View Post
10-4. With those MV inconsistencies, I understand.

Down-range groups and drops are the final say-so for sure. I use my chrono data as an aid to getting there - sometimes with less time and effort than without. I also like the down-range velocity confirmation. Combined with good correlation to drops, it helps ensure the ballistic software inputs and program are providing realistic predicted data at other ranges and under other atmospheric conditions.
If you measure the ES while shooting at 100 yards certainly the group at 100 yards tells you more about the accuracy of the load at 100 yards. But I'd expect the ES to be a better predictor of what the groups would be like at 1000 yards or more than what the group size at 100 yards tells you. That's assuming that neither the rifle or the chronograph have a severe defect.

Bullet drop is proportional of the square of the time of flight so small velocity variations cause a much larger percentage of the group size at long range (many hundreds of yards) than they do at one or two hundred yards.

Wind deflection is proportional to the actual time of fight minus the "vacuum time of flight". (muzzle velocity times distance). Velocity variations do affect wind deflection but only as it directly changes those real and potential times of flight. There is no time squared term related to wind deflection calculation. Generally wind deflection increases over a given distance with decreasing muzzle velocity but there is a dramatic reversal of that trend just below the speed of sound.

Wind deflection uncertainty is determined by the shooters ability to judge the wind over the path to the target and to apply that to it's effect on the bullet trajectory. Velocity uncertainty is related the quality and consistency of the loads, and also the condition of the bore from shot to shot. Either wind estimation uncertainty or vertical stringing can be the limiting factor on group size. The accuracy of a marksman (or rifle) at 100 to 200 yards can be nearly unrelated to their accuracy at 1000 plus yards since the dominating errors may be from completely different causes.

Last edited by LouBoyd; 03-17-2010 at 01:05 AM.
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