What's an acceptable standard deviation?

[DMF13]

I finally got a chance to shoot 10 rounds over a buddy's chrono.

The load is 4.1grains of Winchester 231, 135gr TC Blue Bullets, Federal SPP, and mixed brass, loaded to a OAL of 1.15".

Shot these out of a Glock 19 Gen 5.

Average velocity: 1011
SD: 18.5
avg Power Factor: 136.6

Is that SD okay? I have no frame of reference, and all discussions I've found online on are rifle shooters concerned about precision distance shooting, so their data isn't real relevant to someone loading bulk 9mm on a progressive press for handguns.

Thanks in advance for any info.

Is that acceptable for SD?

[Artemas2]

It's good data to have, but unless you are loading on the extreme ends of pressure or were trying to get every possible ounce of accuracy out of the load, I would not stress too much for general range/practice/practical style shooting.
So long as the rounds are safe, reliable, and accurate enough for their intended purpose, with a margin for power factor requirements(if applicable).

With mixed brass in particular you can drive yourself mad trying to maximize performance

[gringop]

SD is pretty much an indication of the Extreme Spread of sample of shots fired. From my chrono data on 9mm out of a G-19 over the years.

load	avg	high	low	extreme spread	SD
3.8 gr HP-38 WSP MG 124gr FMJ	1001	1011	994	17	6
Blazer Brass 115gr Bulk	1084	1100	1068	32	9
147gr Golden Saber	971	996	932	64	21
124gr FMJ 4.5gr HP-38	1079	1095	1069	26	8

Notice that the SD follows the ES.

If I was loading precision ammo for Benchrest, I would be happy with any single digit SD. For hunting or self defense, I'm good with anything under 20. Plinking or practice ammo, under 30 is Ok with me. I had one load for 45-70 405 gr bullets at 2096 fps that had a SD of 63. I did not choose that load.

Note that all my measurements are 5 shots groups with an Oehler Model 35P. For better precision, 10 or 15 shot groups would be give more accurate results.

Gringop

[358156hp]

As long as the velocities are withing an acceptable (to me) range and accuracy is also acceptable (to me), I just blow off things like std deviation because of the myriad of conditions that could cause it. I used to wonder how primers were tested for consistency. A slightly hotter or colder primer could also throw of SD, as could a different lot of brass.

[Jim Watson]

Standard deviation is a product of mathematical massaging of real quantities.
In the short runs typical of chronograph sessions, it is often about 1/3 of the extreme thread.

A PhD I know preferred Coefficient of Variation (SD as a percentage of the average) and concluded that 1% was “match grade” handgun ammo, 1.5% was good and 2% was OK. Target rifle ammo is more consistent, SD often in single digits.

[Outpost75]

Sample size affects CV. In a 5-shot sample CV is about .28 of the Mean. Useless for assessing ballistic uniformity.
A 10-shot same has a CV about .18 of the Mean. Better, but not conclusive unless you do repetitive samples.

[DMF13]

[Thanks, to for info

Accuracy is compatible to some factory loads I had been using.., and I was most concerned with making power factor if I ever go to a major match.

A PhD I know preferred Coefficient of Variation (SD as a percentage of the average) and concluded that 1% was “match grade” handgun ammo, 1.5% was good and 2% was OK. Target rifle ammo is more consistent, SD often in single digits.

Are you saying I should take the 18.5 SD divided by the average 1011fps, which would be 1.83%? If so, it looks like I might want to make a little effort to improve my process a bit, to get the SD to 15 or less.

[Jim Watson]

I wouldn't bother. Such small runs, the SD and CoV jump around a lot. Unless you are running long strings and are going to start comparing velocity variation with target accuracy, you are OK.

[revchuck38]

I worried about SD when I was still shooting competitively. Going from memory - and I'm sure someone will correct me if I'm wrong - ~65% of your shots will be within +1 SD of your average velocity, and ~95% of your shots will be within +2 SD of the average. I made sure my average velocity was at least 2 SD over the required power factor. Nowadays I just make sure my average velocity is fairly close to my carry ammo, which coincidentally means that it makes power factor anyway.

Other than making power factor, your ammo needs to be safe, reliable, and accurate. An SD of 18 is fine for what you're doing.

[Spartan1980]

I quit looking at SD when I nailed down a cast bullet WST 9mm load that had atrocious numbers. Atrocious compared to factory loads that had better numbers. It grouped just fine on targets, just as good as premium factory ammo. That taught me that for pistols it just doesn't matter. Maybe for olympic shooting but not for the other 99.85% of us.

Rifle is a different matter altogether.