Correlation between SD and accuracy

[BAWunder]

I think the prevailing wisdom is that you want to strive for a low SD. The idea being that the more consistent the round, the more accurate. I was testing out a new load (for me anyway) of 3.2 TG/147 Xtreme. The SD was one of the highest I recall getting, somewhere in the order of 55ish. And yet from an accuracy perspective, it was one of the best I've ever loaded.

So now I'm questioning whether I should even give a rats butt about SDs.

What are your experiences and observations?

BW

[bowenbuilt]

I really don't give a rat's ass what the SD is as long as the round meets PF. I have found in the past 45 years of reloading that SD has nothing what-so-ever to do with how accurate a load is. I always load above the PF for this very reason. I never worry about not making PF and I don't even give SD a thought as long as the group is under 2" who cares. Some of the most accurate loads I have would not pass most people's SD requirement but shoot under 1.5" at 25 yards. Don't get caught up in the SD thing. Load and shoot, stay well above the PF and use the load that is most accurate.

Edited June 16, 2015 by bowenbuilt

[BAWunder]

Thanks, good to know. Why is it then people are so focused on SD?

[HOGRIDER]

IMO, a good explanation:

I teach statistics, and the misuse and misinterpretation of the SD given on chronographs gives me the hives!

Seriously, although the SD can be calculated from 3 shots, it does not mean much until more than 10 (preferable 20) shots have been used in the calculation.

There is a calculation to determine "significance" of the deviation mathematically, but even that is misleading, since it assumes independent measurements. If there is a systematic error, the samples are not independent.

Then, it is also a question of what caused the variations (or deviations). It can range from erratic measuring, different positioning of the powder charge from shot to shot, poor discipline using the chrono, and possibly actual erratic behavior of the powder--which is usually the least likely.

And finally, at distances under 100 yards, it takes a very large change in velocity to move the bullet very far on the target, which makes the SD not very important for most pistol shooting, compared to the other things that can go wrong.

Oh, the calculators on the chronographs are quite capable of doing the math correctly. The problem is that the "conventional wisdom" that small SD ( small changes of speed shot-to-shot) is a major factor in accuracy of pistol ammunition is just not true.

If the SD suddenly went from 20 to 200, I would be looking for what mistake I made in loading or measuring my powder. But an SD of 30 vs an SD of 60 in pistol ammo is in the "so what" range for most purposes,or what we would call a second order effect. When you are trying to shrink a 100yd group from 2" to 1", then we might look at it.

So when you get your 100yd pistol groups down to 2"....

[amokscience]

You care about Standard Deviation if you are loading right at the edge of power factor. I try to have a load where 2 standard deviations is still making power factor. I believe that means that roughly 97.5+% of my rounds will make power factor.

[Hi-Power Jack]

Thanks, Hogrider - very informative. Appreciate it. :bow:

[wingnut]

How do you calculate SD.

[CZinZA]

Don't do it wingnut.....

Some times the chronograph can give you the result directly. If yours doesn't, then type the velocity numbers into Excel and use the function stdev().

To do the calculation by hand is a real pain - Wikipedia will tell you how I imagine. I'm typing with one finger on a small phone screen, so I'm not gonna

[d_striker]

Thanks, good to know. Why is it then people are so focused on SD?

Power factor. Even if you're loading well above PF and you have a high SD, you're risking some ammo being below intended PF.