Standard Deviation (SD)

[cindynles]

If you have four rounds at 3000 fps and one round at 3010 fps you have 4 good rounds and one round ruining your group.
If you look at the mean and the standard deviation you would think your ammo is good when it isn't.

As nobody shoots 100 round groups with 100 feet of variation I don't have an answer for your hypothetical question.
That said your slowest round and your fastest round tell you your vertical spread all else being equal so standard deviation is just a waste of time in my humble opinion.

That's because you don't understand the math behind the statistics. Standard deviation is an indicator of the variance and consistency of your loads. Standard deviation is a predictive statistic, its used to mathematically predict how the rest of the batch is going to perform with a high level of confidence.

Extreme spread is a descriptive value which only tells you about the shots you fired.

[Metal God]

ES only records 2 rounds out of any amount fired . So I agree with others that say SD is more important . Think about it . You can fire 100rds and 99 of them are exactly the same velocity and one is 50fps faster . You would have a ES of 50 and a SD of about 0 . Clearly that one round had an issue and could likely be discounted as bad case prep or other factor . Many people in fact remove a case from there main lot when they have those odd numbers . It's a sign there is something wrong with that loaded round and not the over all load it self .

[bsumoba]

I think you guys are missing the point. You need to apply this to shooting and most of us handloading with any kind of precision do not see ES numbers above 30 or 40 fps and most of us competing will see ES numbers in the teens or less.

His point is that if you have a low ES, ultimately your SD is going to be relatively low. Get your ES as low as you can and your SD will in turn, be low, plain and simple.

Example...

3000,3000,3000,3005,3005,3005,3005,3010,3010,3010, 3010,3000,3000,3000,3000,3000,3010,3010,3010,3030

That 3030 shot is a killer because it is a shot that is 20 fps away from the next highest shot velocity and will most likely result in a bad shot. The SD of this group is around 7.1. Most people would think that is great, but you most likely just had a bad shot. We are trying to have 0 bad shots.

When you tell someone you have a rifle/load that shoots single digit SD, that means nothing IMO. If you tell me you have an ES of 30, I would say you need to work on your load or figure out why you have a flyer that brought your ES to 30. Take away that shot and now you have an ES of 10 fps.

[LynnJr]

Lot

[LynnJr]

Lots and lots of confusion here.
In the example Frank gave in his earlier post about the 100 rounds the Standard Deviation in 1 and the ammo is junk.
Standard Deviation gets lower with a higher round count even though the extreme spread which determines your vertical spread has never changed.

If you don't like your extreme spread you need to improve your reloading practices.

If you don't like your Standard Deviation shoot more ammo and it will drop as Frank has clearly pointed out.

And yes I fully understand the math even though most here don't.

[cindynles]

Lots and lots of confusion here.
In the example Frank gave in his earlier post about the 100 rounds the Standard Deviation in 1 and the ammo is junk.
Standard Deviation gets lower with a higher round count even though the extreme spread which determines your vertical spread has never changed.

If you don't like your extreme spread you need to improve your reloading practices.

If you don't like your Standard Deviation shoot more ammo and it will drop as Frank has clearly pointed out.

And yes I fully understand the math even though most here don't.

You don't fully understand the math if you are making statements like the bold one above. SD is a predictive statistic for a population based on a sample of units from the population being tested. The only change in standard deviation with an increased sample size is that the sample SD becomes more accurate and representative of the population.

Extreme spread is only going be useful if you are going to fire every round over a chrono. SD allows you to take a small sample from your batch and predict how the rest will behave.

[Thoughts]

The only change in standard deviation with an increased sample size is that the sample SD becomes more accurate and representative of the population.

This is correct. SD does not necessarily drop with increased sample size. It might increase, for that matter.

[Metal God]

NO no guys Lynn has spoken , let it be done .

[LynnJr]

Metal Dog
Has it correct. Your extreme spread gives you your vertical spread so if shots 1 and 2 have a 100 fps variation and the other 98 rounds all match shot 1 or shot 2 your extreme spread is your group size.
And yes adding more shots makes your junk ammo look better than it truly is.
Standard Deviation is the Fools Game of reloading accurate ammo.

[LynnJr]

That should read BSumoba has it right.

[pennstater]

What if you're shooting over the screens in Jan., then maybe in May. But, you're going hunting in Sept and shooting one last time over the screens again in Sept? All the SAME loads. Will the SD be the same? Doubt it. I'm thinking LynnJr has it right.

[LynnJr]

Pennstater
What people are missing is it doesn't matter how your best rounds shoot.
It does matter how your worst round shoots as matches are won and lost on your worst round/rounds.

In hunting situations you just don't want that first shot being your worst round.

[bsumoba]

Pennstater
What people are missing is it doesn't matter how your best rounds shoot.
It does matter how your worst round shoots as matches are won and lost on your worst round/rounds.

In hunting situations you just don't want that first shot being your worst round.

Yup...

OP, pay attention to getting consistent ES numbers and you will learn more than paying attention to the SD. SD has its place, but it is not in chronographing precision loads.

Maybe if you want to chronograph bulk factory/handload ammo you can and that might tell you something such as what the general box/case of ammo will produce.

[ar15barrels]

I use a chrono on my ladder tests

What do you expect to learn from that?

The purpose of a ladder test is to find the barrel's charge weight node.
Then after that, you adjust seating length to fine tune the accuracy within the node.

Then you can dig out the chrono to check the ES/SD and see what you have...
Velocity measurements during a ladder test will NOT give you SD or ES figures of any use.

[bsumoba]

What do you expect to learn from that?

The purpose of a ladder test is to find the barrel's charge weight node.
Then after that, you adjust seating length to fine tune the accuracy within the node.

Then you can dig out the chrono to check the ES/SD and see what you have...
Velocity measurements during a ladder test will NOT give you SD or ES figures of any use.

I think the theory is that as you approach nodes, the velocities will flatten out somewhat at the nodes. Never tried this though and can't imagine it really doing this.

I just find a load that produces a similar POI pattern (if developing a load at 100 yards) or vertical similarities (if I have a chance to get out to 300 yards) over a certain grain spread (anywhere from 0.3-0.7 gr spread for most cartridges).