STDEV and Spread targets?

[JTBubbler]

First post, but have been reading as a guest for a while. Thanks for the information.

I have been reloading 7mm Rem Mag for about 2.5 years and about 10-15lbs of powder so just getting started in the grand scheme of things.

I bought a Caldwell chrono last week to learn some more about my loads, and it worked.

My question is this, generally speaking, is there a target for good reloading practices that the chrono can point out as far as STDEV and Spread?

Shooting Nosler Ballistic Tip 150 over 62.0gr H4831 CCI 250 3.340 coal. Slow, but groups better than other loads so far. ELD-X's are on the way with tracking numbers for the next project.

After blowing shades off a few times at 10 feet I moved chrono back to 15 ft and shot the following:

2699, 2694,2706, 2707, 2707
stdev= 5.86 Spread= 13

I know it's a small data set, but it's what I have until I can get back to the range again.

Also, will stdev and spread vary with different powder charges, or should it always be tight, regardless of the load and target pattern if I have good loading practices?

Thanks.

 

[FearNoWind]

Keep the ES low and the SD will take care of itself.
"Low" IMO would be somewhere under 10FPS over a ten round set.
Example:
A ten round string ranging from 2765 to 2785 will give you something like ES 20 SD 6
A ten round string ranging from 2765 to 2775 will give you something like ES 10 SD 4
See the correlation?

 

[dsculley]

While I do notice ES, my main concern is SD. I want to know what to expect in the future as I shoot and the only thing that can provide that data is SD. Assuming a normal distribution, 99.7% of future shots will be within +/- 3 SD's of the average. That means a total spread of 6 SD's. In your case that would be 6*5.86 = 35.16.

When you look at a sample, look at the distribution as well. In your sample, the average was 2703 fps (rounded to the nearest whole number). Now look at the distribution:



MV Distance from Mean
2694 -8.6
2699 -3.6
2706 3.4
2707 4.4
2707 4.4

You will notice that the shots are clustered at the outer extreme of your ES, rather than clustered around the average of 2703. Four of these are within +\- 1 SD. If the 2694 had been 2698, then SD would have dropped to 4.51 & 6*SD would have dropped from 35.16 to 27.03.

If on subsequent trips you run more of this load through the chrony, you can add the future data to this data and process it in a spreadsheet such as Excel to give you a much better idea of what to expect from the load. I always save my data to a spreadsheet and add to it on subsequent trips during load development. I try to get at least 20 rounds logged through the chrony, more if possible.

One more thing you can determine from all this. When you plot your actual numbers, you should have as many above the average as below it. If you find this out of balance either way, then your process is out of control. That is, something in your reloading process is not being done consistently the same way.

In a normal distribution, 68% of the population is +\- 1 SD of the mean. 95% is within +\- 2 SD's of the mean, 99.7% is within +\- 3 SD's of the mean.

One last caveat. Chronographs are susceptible to outside influences due to changing light conditions, etc. While I like to see low SD, I trust my target more than the chronograph. If I am shooting sub .5 MOA groups at 100 yds during development, and this continues when I extend the range then I pay more attention to my target than my chronograph numbers.

To sum it up, I like small numbers. I like small groups better and will trust what the group is telling me versus what the numbers tell me.

FWIW

 

[FearNoWind]

... I like small groups better and will trust what the group is telling me versus what the numbers tell me.

FWIW

Oh yes; I certainly agree with that.
Only have one point to make, Sculley. Even though I take no issue with your approach to the statistical analysis, I'd want the new shooter to know that there is more than one way to look at the numbers. That's why I focus on ES rather than SD. I know that if I keep my ES below 10, my SD is going to be within acceptable range. As I stated, my ES objective is 10.
Sample:
2699,2694,2706,2707,2707 = ES 8 and SD 5.86
Sustituting:
2700,2701,2706,2707,2707 my ES drops only by 1 (=7 or about 12%) but my SD drops to about 3.5 (or about 41.6%)
As I carry that out further:
2700,2702,2704,2706,2708,2710,2712,2714,2716,2718 my ES goes out of sight at 18 but the SD remains looking pretty good, about as good (at 6) as it did in the first example at 5.86
So even by collecting large amounts of data and adding it to my spread sheet over time if I focus on SD when I have a problem I run the risk of allowing the minor variations in the results reflected from the data to influence my decisions for load adjustment.
I know instincively however that if my ES remains below 10 my SD will take care of itself.
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