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Es/sd
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ShtrRdy
Well-Known Member
ES - stands for extreme spread. This pertains to the velocity measured from a group of cartridges and describes what the difference is between the highest velocity and lowest velocity.
SD - stands for Standard Deviation. This also applies to the velocity measured from a group of cartridges. It's a statistical estimation of the probability that the velocity of the next cartridge fired will be within some range of velocity.
These sorts of things are mostly important for long range shooting because a different velocity will result in the bullet impacting higher or lower.
SD - stands for Standard Deviation. This also applies to the velocity measured from a group of cartridges. It's a statistical estimation of the probability that the velocity of the next cartridge fired will be within some range of velocity.
These sorts of things are mostly important for long range shooting because a different velocity will result in the bullet impacting higher or lower.
dok7mm
Well-Known Member
What brand of chronograph are you using? Most all will give you the ES & SD of your shot string. The trick is picking the powder, charge weight, primer and seating depth to lower both numbers enough to deliver the round to round accuracy you are looking for.
benchracer
Well-Known Member
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- Dec 18, 2009
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- 1,659
Here are step-by-step instructions to calculate SD:
https://www.mathsisfun.com/data/standard-deviation-formulas.html
https://www.mathsisfun.com/data/standard-deviation-formulas.html
If you have MS Excel spreadsheet, it's one of the standard formulas. Probably in others too.
dsculley
Well-Known Member
ES - Extreme spread - the difference between your fastest and slowest shot in a string. While helpful, this is not nearly as good a tool as SD (Standard Deviation).
SD - Standard Deviation - a predictor of the range of velocities of all your shots based on a sample. (This is the most important of the two numbers)
Example: Assumptions for this example: ES = 9, SD = 2.5, Avg Velocitiy = 3000 fps
The ES tells you that on this batch of ammo, you did a good job and total variation in THIS string was low.
Using SD for this string as 2.5 and assuming a normal distribution for your ammo, you can expect that 68.2% of your rounds will fall within 2 standard deviations (+1 & -1), 95.4% of your rounds will fall within 4 standard deviations (+2 & -2), 99.6% of your rounds will fall within 6 standard deviations & 99.8% will fall within 8 standard deviations.
3000 fps avg
68.2% will be between 2997.5 & 3002.5 (ES = 5)
95.4% will be between 2995 & 3005 (ES = 10)
99.6% will be between 2992.5 & 3007.5 (ES = 15)
99.8% will be between 2990 & 3010 (ES = 20)
As you can see, the expected extreme spread (> 20 fps) of your total population is more than twice ES of your string. Using SD, you can use ballistics programs to see how this will affect your point of impact in determining if this load is acceptable for your purposes.
I think any shooter on this forum would love to have an ES of 9 and SD of 2.5, but many shooters don't really know how to interpret the figures, especially SD. As hand loaders, our goal is for consistency in our processes so that we will have consistency in our results.
I hope this helps.
Dennis
SD - Standard Deviation - a predictor of the range of velocities of all your shots based on a sample. (This is the most important of the two numbers)
Example: Assumptions for this example: ES = 9, SD = 2.5, Avg Velocitiy = 3000 fps
The ES tells you that on this batch of ammo, you did a good job and total variation in THIS string was low.
Using SD for this string as 2.5 and assuming a normal distribution for your ammo, you can expect that 68.2% of your rounds will fall within 2 standard deviations (+1 & -1), 95.4% of your rounds will fall within 4 standard deviations (+2 & -2), 99.6% of your rounds will fall within 6 standard deviations & 99.8% will fall within 8 standard deviations.
3000 fps avg
68.2% will be between 2997.5 & 3002.5 (ES = 5)
95.4% will be between 2995 & 3005 (ES = 10)
99.6% will be between 2992.5 & 3007.5 (ES = 15)
99.8% will be between 2990 & 3010 (ES = 20)
As you can see, the expected extreme spread (> 20 fps) of your total population is more than twice ES of your string. Using SD, you can use ballistics programs to see how this will affect your point of impact in determining if this load is acceptable for your purposes.
I think any shooter on this forum would love to have an ES of 9 and SD of 2.5, but many shooters don't really know how to interpret the figures, especially SD. As hand loaders, our goal is for consistency in our processes so that we will have consistency in our results.
I hope this helps.
Dennis
that would be exceptional
I have one load that averages ES 11 and SD 4 and I am smiling. best I have gotten so far
DocB
Well-Known Member
Dennis, are the percentage ranges you used for 'normal distribution', ie. 68% (+1,-1) et.al standard for determining standard deviation 'spread'? Sorry if this is worded incorrectly, this is new to me, I have the theory I think, just not the correct way of stating it.
Thanks,
DocB
To benchracer: Thanks for the link now I can do the math myself if need be.
To Dennis: With your percentage info I better understand how to use SD after I arrive at what that # actually is.
My hats off to both of you, thanks for the help!
To Dennis: With your percentage info I better understand how to use SD after I arrive at what that # actually is.
My hats off to both of you, thanks for the help!
dsculley
Well-Known Member
The numbers I used are representative of a normal distribution. The percentages don't change. Most people round the me to the nearest whole number, but I used to the nearest tenth.
The numbers I used were random and used because they were easy to work with as I wrote. As I said, most (all) of us would be very proud to have loads like that.
The point of the exercise was to highlight the difference of SD and ES. You can't project the ES of a string of shots to the entire population of rounds that you will load. For fun, you can use higher (more normal?) numbers for SD to see just how much variance there is.
When you get a load really dialed in, run a 10 shot string across you chrony to see where you are. Use the above information to find the outer limits of your mv based on the sd, then run those 2 numbers through your ballistics program.
Dennis
The numbers I used were random and used because they were easy to work with as I wrote. As I said, most (all) of us would be very proud to have loads like that.
The point of the exercise was to highlight the difference of SD and ES. You can't project the ES of a string of shots to the entire population of rounds that you will load. For fun, you can use higher (more normal?) numbers for SD to see just how much variance there is.
When you get a load really dialed in, run a 10 shot string across you chrony to see where you are. Use the above information to find the outer limits of your mv based on the sd, then run those 2 numbers through your ballistics program.
Dennis
