SD and ES question

[Steave]

Obviously one want to have the ES and SD as low as possible, but what is a good and realistic goal to shoot for?

Thanks much!

 

[15Tango]

Short answer, single digit. It's possible and realistic.

Long answer - depends. If you're shooting something like 6mm 115 Dtac at 3200 FPS, you know, something that has a good BC and a respectable velocity and you're only shooting at 200 yards, single digit SD doesnt' really matter a whole lot. I think for the better bullets and cartridges out there an SD in the teens will suffice for 400 yards and under, maybe even 600.

If you use an online ballistics calculator or something like Sierra's program you can figure it out for yourself if what you're getting is good enough for you. If you've got a DTAC running at 3000 FPS with an ES of 30 (2985-3015 FPS) you can see the numbers for yourself:

DTAC @ 2985 @600 yards =70.2" drop
DTAC @ 3015 @600 yards =68.58 drop
That means with an ES of 30 FPS that DTAC at 600 yards will group at 1.62" at best, all other things being equal. That's the theory. If you're a good shot and shooting F-Class that's still enough to keep you in the 10 ring if not the X.

At 1000 yards however that 30 FPS ES looks like this:
2985 FPS = 269.75"
3015 FPS = 263.55
It's equating to 6.2" of shot spread now, over half a minute where at 600 a 1.62" spread was under .3 MOA.

I hope that sort of helps and answers your question . . . or something. Those are just my thoughts and feelings on the matter anyhow.

Wayne

 

[bozo699]

well put Wayne.
Wayne.

 

[RonAKA]

You have to keep the numbers in perspective. Standard Deviation is a valid statistical tool for predicting the range of future values. See graph below. If you take your average velocity and add and subtract one standard deviation, then 68.2% of future shots under the same conditions should be in that range. If you go +/- two standard deviations confidence increases to 95.4%.

You need about 10 samples for the SD to start to become realistic, and ideally 20-25. So if you use enough samples then SD is probably a very good measure of the velocity range you can expect.

Extreme spread on the other hand, has no statistical significance that I am aware of. It is just a post mortim on what happened in the past. If you had a large group and the ES was also large, then you may have an explanation. SD self compensates for sample size, but ES does not. So if you have an ES number you have to compare it to the sample size to get a gut feeling as to what it means.

My thoughts are that if you are using the data to measure quality of your loads, then SD is the number to look at, and only if the sample size is large enough. Does low SD translate to small groups? Not in my experience at 100 meters. It may be much more significant at 1000 meters.

 

[dreever]

I try for ES numbers in the single digits. However if I can't get them I try the load on paper a few times at the longest yardage i can get at my home range. If it groups well I don't worry about it to much.
In my .308 with the Berger 185 gr. BTHP Long Range seated to a mag length of 2.885 the best ES I could come up with was 22 fps. That load will shoot four inches and under at 840 yards when I do my part so I'm not to concerned with the ES in the low twenties.

Danny

 

[Steave]

Thanks guys, 'preciate the info!