What Standard Deviation should I expect?

[Jeff686]

I chrono'd some new loads yesterday. 38 Super Comp, Longshot, 124gr at various amounts of powder.

How much standard deviation is normal? What should I strive for?

Will I get some as the gun heats up?

My best batch had a SD of 2.4 fps, my worst 16 fps.

Any insight?

[G-ManBart]

Anything under 10 is really good. 2.4 is crazy good...new brass right? Even at 16, that's totally fine. If it was 30 or more I'd start looking to see what was going on, but even in the 20s is good enough for what we do.

[Flatland Shooter]

I chrono'd some new loads yesterday. 38 Super Comp, Longshot, 124gr at various amounts of powder.

How much standard deviation is normal? What should I strive for?

Will I get some as the gun heats up?

My best batch had a SD of 2.4 fps, my worst 16 fps.

Any insight?

Like Bart said, 2.4 fps is crazy good (I don't think I've every had a load that good! But then I'm new at reloading. Just started in 1965.) and 16 fps is not bad.

If shooting these loads in USPSA competition, the lower the SD the closer you can load to the threshold power factor without risking the surprise of chronoing minor at a major match. Check through the forum and you will find some people (who are a whole lot smarter than me) discussing the merits of SD versus PF.

The forum gurus have the consensus that one should load to at least three standard deviations faster than the minimum velocity for required for major. With the 124 gr bullet you need a velocity of 1331 fps to make major. With a consistent SD of 5 or less, you should be safe loading at 1346 fps (1331 + 3 x 5 = 1346). This equal a target PF of 166.9.

So, increasing the SD means you must increase the velocity and PF to assure you make major at a match. (Increase your SD to 30 and you will need a PF of 176.2 to meet the above guidelines.)

What SD should you strive for? Both loads you chrono'd look good so you've met that goal.

Will this change with a hot barrel? Don't know and don't care. At a major match with a chrono, they will only fire a few rounds to get a reading. Not enough to get the barrel hot.

HTH

Bill

[enoon]

Standard deviation is not simple. Sample size should be considered. Sample mean variations should be considered, if appropriate. Confidence levels. It goes on and on and probably is not all that interesting or worthwhile to spend too much time on it. Unless one enjoys the exercise or gains confidence from the exercise. Chronographs and techniques also introduce/reduce variance.

Short version. If standard deviation is a calculation you feel confident using, the sample size should probably be at least 10 and better if it is 15-20. Use randomly selected rounds from throughout a production run(s) of your intended load. Sometimes people seem to try to create a small standard deviation to justify a load rather than test the load.

I have never really isolated the effect of hot barrels. I have noted that the temperature of the round (from resting in a hot chamber) can have an affect.

As an aside, standard deviation of the velocity of a load can be an indicator of accuracy. The target will tell you for sure.

[warpspeed]

enoon pretty much nailed it.

And if you want to compare one load to another, samples need to be equal.

5 rds is not enough. I'd say 50 is a waste of ammo unless you are playing a different game than USPSA.

I'd use a sample no less than 10 but probably more like 20 if I really cared. BTW - I don't.

The results show up on the target. The chrono just tells me if the shotgun pattern is caused by velocity or something else.

Always shoot for group over the chrono. A single digit SD with a 5" group at 25yds is worthless and a 25 fps SD with 1.5" @ 50 yds works just fine.

[wide45]

I chrono'd some new loads yesterday. 38 Super Comp, Longshot, 124gr at various amounts of powder.

How much standard deviation is normal? What should I strive for?

Will I get some as the gun heats up?

My best batch had a SD of 2.4 fps, my worst 16 fps.

Any insight?

Like Bart said, 2.4 fps is crazy good (I don't think I've every had a load that good! But then I'm new at reloading. Just started in 1965.) and 16 fps is not bad.

If shooting these loads in USPSA competition, the lower the SD the closer you can load to the threshold power factor without risking the surprise of chronoing minor at a major match. Check through the forum and you will find some people (who are a whole lot smarter than me) discussing the merits of SD versus PF.

The forum gurus have the consensus that one should load to at least three standard deviations faster than the minimum velocity for required for major. With the 124 gr bullet you need a velocity of 1331 fps to make major. With a consistent SD of 5 or less, you should be safe loading at 1346 fps (1331 + 3 x 5 = 1346). This equal a target PF of 166.9.

So, increasing the SD means you must increase the velocity and PF to assure you make major at a match. (Increase your SD to 30 and you will need a PF of 176.2 to meet the above guidelines.)

What SD should you strive for? Both loads you chrono'd look good so you've met that goal.

Will this change with a hot barrel? Don't know and don't care. At a major match with a chrono, they will only fire a few rounds to get a reading. Not enough to get the barrel hot.

HTH

Bill

That would work if you knew that your chrono, and the match chrono both read the same velocity, and temperature, humidity, and altitude would not be a factor. Dream on.

Most load to 5 over the minimum power factor. Say average 170, if minimum is 165. Then make sure that the lowest of a reasonably large sample is not under 165. Bump up your average a little if your extreme spread is large, on an otherwise good load.

No need to make it complicated.

[Scout454]

I think you guys are confusing Standard Deviation (SD) with Extreme Spread (ES). Figuring SD is much more complicated and is probably less important in a handgun than a rifle. While an SD of "2.4" is outstanding in a rifle and almost unheard of in a handgun, it is not nearly as important for Power Factor as ES is. If you load close to minimum PF and have a 30 to 40 FPS spread, you could easily drop below major (or minor). To get an SD of 2, your ES would be less than 5 FPS. Possible, but how many rounds were run over the chrono from that batch and did you check it again after loading another 100 rounds with the same load. Any single digit SD is acceptable for both handguns and rifles. If you can load to an ES of "2.4" FPS, as other posters have stated, check at least 10 rounds to make sure it holds - and then check it again.