# ES and SD

#### Waknstak

##### Well-Known Member
So I got that ES is difference from slowest to fastest...... Is sd the average of the difference of speed from shot to shot then? Math is ot my Forte..... Thanks

#### NRF

##### Member
So I got that ES is difference from slowest to fastest...... Is sd the average of the difference of speed from shot to shot then? Math is ot my Forte..... Thanks
Standard deviation is the average of all readings calculated. Chronograph 5-20 shots and calculate the average for your dope chart. Extreme spread is the difference from lowest to highest chrono reading. When I reload I try to stay under 20 FPS ES or I keep tweaking the load till I reach that.

Thanks alot

#### Greyfox

##### Well-Known Member
SD isn’t really an average of the velocities of shots fired, as that would be the “mean”(or average), the number you would plug into your ballistic calculator for MV. This is the sum of all your velocities divided by the number of shots. ES is the extreme spread between the lowest and the highest velocity in a string of shots. SD is “standard deviation” which is a formula that calculates the distribution of the individual shots and produces a value that represents a confidence level where 95% of your fired shots will fall within a given range. The SD (as shown in the output in chronographs like the Magneto Speed, Oehler, etc.) can generally be multiplied by 4 to give you an approximation of the velocity spread where 95% of your shots will fall within this range. For example, an SD of 5 is approximately 20FPS, where you can expect 95% of your shots to fall within this velocity range. For my LR hunting to 1000 yards I shoot for an SD of 5-10, or an ES that’s less then 30FPS. This may sound high to some but it uses a sufficient sample of shots of shots(+20), and applies to the range of temperatures I expect to hunt. This value will keep my shots in the vital area of a whitetail/antelope out to 1000-1200 yards with my particular load, other factors considered.

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#### Waknstak

##### Well-Known Member
So sd is a number your chrono ***** out then?

#### LongBomber

##### Well-Known Member
I see lots of guys talking (esp on FB pages) about SD and ES. Most of the time it seems as though they are only using 5 shots to derive the numbers. To really have any decent stats you need over 30 shots to get the SD number to really be meaningful. A 5 shot population just doesn't give enough info.

#### phorwath

##### Well-Known Member
Agree. But five is better than 3. Not as good as ten. It gives you an idea on how uniform the MV is from shot to shot.
It's all relative, in my opinion. 100% certainty is only necessary for the full blown obsessive-compulsive. And I'm only about 75% OCD.

#### Canhunter35

##### Well-Known Member
Google bell curve, standard deviation is the big middle part where most of ur shots will fall.

#### Chippewa

##### Member
Found this... If you feel likemaking your old math teachers proud!

To calculate the Standard Deviation of those numbers:

Work out the Mean (the simple average of the numbers)
Then for each number: subtract the Mean and square the result.
Then work out the mean of those squared differences.
Take the square root of that and we are done!

#### brentc

##### Well-Known Member
I'm trying something a little different with my newest project. My sample consists of only cold bore velocity readings over a range of environmental temperatures to see how much temperature affects my first shot velocity with RL26 in a 300 WM.

It has been eye opening. For those of you worried about temp swings with RL26, you really shouldn't be. My Berger 215 load has shifted 2 fps with cold bore shots ranging from 28° up to 56° so far.

#### NRF

##### Member
I’m still shooting RL22 Out of my 300 WM! Have you tried both? I’m getting an ES of 12 FPS out of 20 rounds with RL22

#### Rhovee

##### Well-Known Member
I’m just to anxious to send more round down range that I have a hard time waiting between shots. I have noticed when leaving rounds in the chamber my FPS climbs as you would expect. I really like the idea of cold bore shots, as this is really what’s important. I am going to start doing that.

#### MagnumManiac

##### Well-Known Member
When calculating SD in my rifle(s) I run 3 20 shot strings at DIFFERING days and conditions in match like order on various practice days at different ranges.
This is the ONLY way I feel comfortable using a particular load for 1000mtr comp.
All of my comp loads run ES of 5-8fps, but a little more really makes little difference vertically, wind plays more of a zephyr at most ranges here.

You can look at SD as how uniform your load is in generating consistent start pressure, without this, your load is erratic and will have high ES regardless of how tight it may group @100-200yrds.
It must be understood that a high SD/ES number does not mean a load is inaccurate, it only means that beyond 500-600yrds the vertical dispersion is too large for true precision shooting.
I have a 25-06 load that prints 3/8MOA @ 300mtr and less that has an ES of 80fps. Not ideal beyond about 400mtr, but awesome @ 100mtr.
This demonstrates that barrel time/harmonics have a direct influence on accuracy, this load, even with it’s large ES is coinciding bullet exit at 2 points that match in the oscillation and, often when a load produces 2 distinct ‘groups’ in a string from barrel harmonics being different, this is the cause.

Cheers.