SD ???

[Texas Solo]

What does SD tell us? Why is it useful info, and how do we use it? ES seems more important to consistent loads to me. My 308 load has an average of 8 ES & under 10 SD. I know that's pretty good. But I don't know how to use the SD information.

 

[piie]

In simple terms: ES shows the delta of your highest and lowest velocity, regardless of how many shots you measure, while the SD gives an indication of how close to the average/mean of the entire string your shots were- in effect “balancing out” the outliers. Example, you may have an ES of 10 for 10 shots, but 9 out of the 10 shots were 10fps, and one shot was 1fps, which would give you a higher SD than having 1 shot at 1fps, 1 shot at 10fps, and 8 at 5fps (the mean)
You would prefer the latter.

 

[pdog2225]

What does SD tell us? Why is it useful info, and how do we use it? ES seems more important to consistent loads to me. My 308 load has an average of 8 ES & under 10 SD. I know that's pretty good. But I don't know how to use the SD information.

It tells you the consistency of the rounds you are shooting. It does not tell you how accurate your ammo is. So you have A chrono and measured your load, at 100yds and you have the numbers. Run two simulations on your ballistics program, one for the highest velocity and one for the lowest. Compare the two sets of numbers at a farther distance than you normally shoot, say 1400 yds or less.Check the amount of each at that distance, multiply the difference times the moa and you will have the distance of dispersion of the theoretical group.All the other rounds should be in between them. For long range (whatever that is) you want the lowest possible ES/SD and a tight group. That is why lower is better. Pdog2225

 

[BoydAllen]

If you do a little research, to have a credible SD you need a larger sample than most shooters use. The problem is that chronographs will generate a number from any number of shots. My advice is to stick with ES. When I see someone talking about the SD for three or five shots, I am pretty sure that they could not tell us how to calculate that number if their life depended on it, or what an appropriate sample size is (20 OK, 30 Better), but because chronographs generate SDs from ridiculously small samples, and the prevalence of those numbers in internet discussions, we will continue to see this.

 

[Straightshooter1]

What does SD tell us? Why is it useful info, and how do we use it? ES seems more important to consistent loads to me. My 308 load has an average of 8 ES & under 10 SD. I know that's pretty good. But I don't know how to use the SD information.

None technically speaking, SD's simply tell me how well/consistent I'm loading my cartridges (nothing more). ES's don't tell me much as it's nothing more than a measure between just two velocities regardless of how many you've shot. Having a ES of 8 with an SD less than 10 is very strange. With an ES of 8 I'd expect an SD to be something like 2.

 

[6Creed73]

Extreme spread is the measurement of two shots in the string, SD is the measurement of all of the shots fired in that string. I'm not sure how you can have an SD larger than your ES since ES is the "extreme" spread. Also, you mention your ES is an average of 8...ES is not an average, it's the extreme difference in velocity between two shots. You may want to recheck those numbers.

I look at both, but tend to use SD as an indicator of how good my load is. I don't put a lot of stock in the data though unless it's at least 10 shots, preferably 20. I've come across people who pay no attention to ES/SD, but they typically aren't shooting long distances. At 1000 yards, a bad spread will put you off target making you dial your turrets, which will put you off target for the next shot and chasing your tail.

It's also a ruler for you to use to improve your loading practices. Good ES/SD indicates you are doing things properly at the bench.

 

[Coyotefurharvester]

And minimal velocity variation require very consistent bullets(weights and bc) in order to see minimal vertical dispersion on targets.

 

[Zero333]

I take ES/SD seriously ONLY when they're from a 10+ shot string. Otherwise they're toilet paper.
WAY too many times have I seen ES #'s less than 3 fps from a 3 or 5 shot string. Take a 10+ shot string to see things closer to the truth.

Many times I've personally shot a 5 shot group that had ES of 1 or 2 fps. I sure as hell would never consider the load to have exceptional ES. I just let the target tell me what I need to know.

 

[Texas Solo]

Thanks guys. I did mess up the stats. I went back and checked my notes.
ES was 8, SD was 4. I said under 10, but I see that was pretty vague.
For the record, I generally start load development with 5 rounds of each. When it's narrowed down, I go to 10.
The 8/4 was my best load for accuracy as well as stats. .250" at 100.
308, IMR4064, lapua scenar 185, 30"
1/10 barrel.

 

[6Creed73]

Thanks guys. I did mess up the stats. I went back and checked my notes.
ES was 8, SD was 4. I said under 10, but I see that was pretty vague.
For the record, I generally start load development with 5 rounds of each. When it's narrowed down, I go to 10.
The 8/4 was my best load for accuracy as well as stats. .250" at 100.
308, IMR4064, lapua scenar 185, 30"
1/10 barrel.

8/4 are really good spreads. My best load is 11/5 over 20 shots when the temp was about 80 degrees. I'm going to do another 20 shot string this winter to see how/if it changes in colder weather.

 

[RegionRat]

And just to be done with this, you are now aware that small sample stats can swing so far that by themselves they are not reliable.

That is not to say "don't even look" but it does say they don't stand by themselves so plan to bolster the sample if you suspect the recipe is worthy and watch close.

Happy New Year and Best Wishes for 2024!

 

[BoydAllen]

None technically speaking, SD's simply tell me how well/consistent I'm loading my cartridges (nothing more). ES's don't tell me much as it's nothing more than a measure between just two velocities regardless of how many you've shot. Having a ES of 8 with an SD less than 10 is very strange. With an ES of 8 I'd expect an SD to be something like 2.

Targets see the effects of ESs particularly at long range, not so much, if at all at close range.

 

[CharlieNC]

Not much statistical work involves ES (range) but focuses on the SD. The ES increases with sample size, so is not as straightforward to determine a reliable answer such as SD which begins to stabilize with n=20.

 

[Straightshooter1]

Targets see the effects of ESs particularly at long range, not so much, if at all at close range.

Even at long range, it just doesn't seem very important when a single flyer determines a large ES and ignores the rest of the group. This is why I feel Mean Radius tells me a lot more than ES's. Certainly, small ES's are better than larger ones.

 

[BoydAllen]

Even at long range, it just doesn't seem very important when a single flyer determines a large ES and ignores the rest of the group. This is why I feel Mean Radius tells me a lot more than ES's.

Yes but in competition mean radius is not the method used. I was speaking based on how sanctioned matches are done.