Extreme Spread, Standard Deviation

[Jammer Six]

After reading a few of the threads here, there are three things that strike me when considering the standard deviation and extreme spread of muzzle velocities when reloading.

One is that they do not measure accuracy, they measure consistency. (Some have called it precision.)

Two is that while they are both desirable from a consistency point of view, they are not the goal when you point a weapon downrange-- the goal is a small group.

And three is the law of large numbers seems to say that while these are interesting, they are not accurate until thousands of data points have been established per load.

Since I don't intend to load a thousand rounds of a load that I may reject, or a thousand rounds of a load that consistently gives large groups, I'll probably never know what the true extreme spread is or the true standard deviation is of a load that I reject early in testing.

Therefore, I regard both these metrics as a curiosity until I have a thousand points of data (or more) with a given load.

 

[ireload2]

1. They are better than no information at all and it is possible that they will predict vertical dispersion at long range.
2. A statistically valid sample size in most statistics references is 30

You cannot know the true extreme spread or sigma is unless you chronograph 100% of the rounds that you shoot.

If you get a load that varies by 300 fps just go ahead and ignore the ES.....

 

[Jammer Six]

Haven't seen that yet, but I have seen a pistol load that varies 185 fps. That's pretty high, considering it's 700 fps load in the first place.

I haven't tested the load for accuracy yet, and after getting my LabRadar ironed out, I intend to measure MV on everything when I test it for accuracy.

 

[dmoran]

After reading a few of the threads here, there are three things that strike me when considering the standard deviation and extreme spread of muzzle velocities when reloading.

One is that they do not measure accuracy, they measure consistency. (Some have called it precision.)

Two is that while they are both desirable from a consistency point of view, they are not the goal when you point a weapon downrange-- the goal is a small group.

And three is the law of large numbers seems to say that while these are interesting, they are not accurate until thousands of data points have been established per load.

Since I don't intend to load a thousand rounds of a load that I may reject, or a thousand rounds of a load that consistently gives large groups, I'll probably never know what the true extreme spread is or the true standard deviation is of a load that I reject early in testing.

Therefore, I regard both these metrics as a curiosity until I have a thousand points of data (or more) with a given load.

All that you wrote in terms of velocity data rationality is as radical left wing as I've ever read. So extreme in rationality that its laughable, but is very consistently fitting of the your nature !.!.!

Realism is what myself and most like to get from discussion, and with that will bid your lobby good day.
Donovan

 

[Barrel Nut]

After reading a few of the threads here, there are three things that strike me when considering the standard deviation and extreme spread of muzzle velocities when reloading.

One is that they do not measure accuracy, they measure consistency. (Some have called it precision.)

Two is that while they are both desirable from a consistency point of view, they are not the goal when you point a weapon downrange-- the goal is a small group.

And three is the law of large numbers seems to say that while these are interesting, they are not accurate until thousands of data points have been established per load.

Since I don't intend to load a thousand rounds of a load that I may reject, or a thousand rounds of a load that consistently gives large groups, I'll probably never know what the true extreme spread is or the true standard deviation is of a load that I reject early in testing.

Therefore, I regard both these metrics as a curiosity until I have a thousand points of data (or more) with a given load.

You're kidding right? Based on your way of thinking, should I ignore neck tension because they don't award trophies for "Best Neck Tension"?

 

[CharlieNC]

Thousands of data points? I suggest you get your information from something other than a shooting site.

 

[Jay Christopherson]

he casteth a wide net

 

[500Stroker]

You Boy's play nice.
Left, right or down the middle, everyone is entitled to his or her opinion, at least here in the good old USA.

 

[dmoran]

^^^^ +1
Agree... but in the creation of this thread, I see it as a "lobby", not an opinion.

Donovan

 

[gilmillan1]

Jammer Six,

You are thinking that small groups and ES/SD are two independent animals. Consequently, you tend reach a conclusion that you have to abide by one or the other. The reality is that when you are reloading you are seeking for the perfect balance between a small group and good ES/SD. The group and the ES/SD will have to be sacrificed in order to reach that perfect balance. You cannot focus solely on ES/SD because you will not achieve that perfect group, and vice-versa, you cannot focus only on the group because that perfect group may not be in the powder node. If you are not in the powder node, you will have some great days and some horrible days, depending on your shooting conditions. You need that balance between both.

 

[Darrel Martin]

Even though we use small samples,it still gives you useful information in judging loads against one another.It does not require burning out the barrel to judge that one load may be better at long range.If it did it would be pointless as you would then need to start on a new barrel in an endless quest.
You can use other data, like the target, but ES and SD are still useful and can be measured at the same time with little extra effort.

 

[500Stroker]

^^^^ +1
Agree... but in the creation of this thread, I see it as a "lobby", not an opinion.

Donovan

A well made Good Point. How so often we forget what this Forum is really here for.
Sorry for gett'n off topic.

 

[Jammer Six]

I can see that. I eliminate far more loads than I use, and I eliminate them for a variety of reasons.

In a load that I'm going to eliminate for large groups, it seems to me that ES and SD don't tell me anything that would save it. However, what I'm not clear on is given a load that shoots reasonable or small groups, when I'm making the decision to eliminate it or continue testing it, what do these metrics tell me then?

 

[Jammer Six]

he casteth a wide net

Maybe he really believes that.

 

[bobinpa]

I can see that. I eliminate far more loads than I use, and I eliminate them for a variety of reasons.

In a load that I'm going to eliminate for large groups, it seems to me that ES and SD don't tell me anything that would save it. However, what I'm not clear on is given a load that shoots reasonable or small groups, when I'm making the decision to eliminate it or continue testing it, what do these metrics tell me then?

This is simple.....

ES or extreme spread is the distance from the center of one bullet hole to the center of the rest of the bullet holes in the same group, so, if you have a high extreme spread, the group is big. It seems to me that most guys on this forum want to keep the groups under 1 moa, which is approximately 1 inch at 100 yards. If you want to keep your grouping under 1 inch you will want to have an extreme spread that is measuring 1.00 or less and a standard deviation of 0.5 or less. As you can see by my example, if you want to shoot small groups, a high extreme spread is bad!

SD or standard deviation is the distance from the center of the aim point to the center of each bullet hole. The higher the standard deviation, the further the hole is from the aim point, thus creating a large group, usually. This is were it gets tricky..... I say usually because all the bullets could go to the same side of the aim point which is rare but possible. If this does happen and as I said it rarely does, but if it does, you could end up with a small group (which is good unless you want a large group) and a high standard deviation (which is bad unless you want a large group), which would just show the shooter that the gun sights need to be adjusted to make the deviation line up with the aim point even though the deviation was standard.

I hope this helps!

 

[SheepDog]

Jammer,
Extreme spread is the difference in fps from the fastest to the slowest shot in a test string. While at long distance it can cause larger groups a load with a high extreme spread can also shoot small groups. Sometimes that barrel twitch works for you.

 

[Jammer Six]

Jammer,
Extreme spread is the difference in fps from the fastest to the slowest shot in a test string.

ES or extreme spread is the distance from the center of one bullet hole to the center of the rest of the bullet holes in the same group, so, if you have a high extreme spread, the group is big.

SD or standard deviation is the distance from the center of the aim point to the center of each bullet hole.

I think that part of the problem is that we're talking about two different things.

In my first post, I specified that I was talking about ES and SD in muzzle velocity.

I measure the targets with the free version of On Target, and use the mean of the group size to compare loads.

I'm trying to determine if the ES and SD of muzzle velocity of a load that doesn't group well tells me anything.

I'm starting to see that both would matter if I took a load from 200 yards to 600 yards, that makes sense to me.

I hope this helps!

It all helps, one way or another. Thanks!

 

[Barrel Nut]

This is simple.....

ES or extreme spread is the distance from the center of one bullet hole to the center of the rest of the bullet holes in the same group, so, if you have a high extreme spread, the group is big. It seems to me that most guys on this forum want to keep the groups under 1 moa, which is approximately 1 inch at 100 yards. If you want to keep your grouping under 1 inch you will want to have an extreme spread that is measuring 1.00 or less and a standard deviation of 0.5 or less. As you can see by my example, if you want to shoot small groups, a high extreme spread is bad!

SD or standard deviation is the distance from the center of the aim point to the center of each bullet hole. The higher the standard deviation, the further the hole is from the aim point, thus creating a large group, usually. This is were it gets tricky..... I say usually because all the bullets could go to the same side of the aim point which is rare but possible. If this does happen and as I said it rarely does, but if it does, you could end up with a small group (which is good unless you want a large group) and a high standard deviation (which is bad unless you want a large group), which would just show the shooter that the gun sights need to be adjusted to make the deviation line up with the aim point even though the deviation was standard.

I hope this helps!

Unfortunately it doesn't help because not only are you talking about something completely different from the subject of this thread, what you say is not quite correct either. My intent is not to be argumentative, but your post has the potential to confuse beginners.

In this thread, we're discussing muzzle velocity, not group size or deviation from the aim point.

For our purposes, the ES is the difference between the fastest and slowest round in a particular group.

The SD (Standard Deviation) is simply the square root of the variance in MV. In simple terms, it's a measure of how spread out the velocity measurements are.

What you're talking about is how bullets group on the target. The distance between the two most remote bullet holes in a particular group is usually called the Center-to-Center measurement, or Group Size. It's common to measure it in inches, millimeters, or MOA. Although it might not be technically incorrect to call this ES, we who shoot targets reserve that term for discussing muzzle velocity, not group size.

What you describe as SD is not something typically measured in target shooting. The closest to your statement about SD is what we call "Offset Distance", which is how far the center of the group is from the point of aim. Most people don't even calculate this. Or perhaps you have SD (or ES) confused with the "Average to Center" or "Mean Radius" measurement which is an average measure of how far each bullet hole is from the center of the group. ATC or Mean Radius is a measurement related to Group Size but it tells us a good deal more about how the bullets hit the target. For example a ten-shot group scattered around inside a 6 inch circle is not as desirable as a ten-shot group where 9 bullets go into one hole and the 10th hits 6 inches away. Both are 6" groups, but the second group is obviously better. A look at A.T.C. (Mean Radius) will clearly reveal the difference, but since it's tedious to calculate by hand, most folks don't measure it unless they use a computer based measuring and scoring program. On Target markets one such program; some versions are free.

 

[P72]

Playing around with some numbers that I randomly derived but are sort of realistic that might occur from my .204 R with 40 Vmax's

3704,3716,3699,3685,3690,3701,3699,3705,3723,3750 ---10 shot velocity/sample size

then

3704,3699,3699,3685,3712 -- 5 shot velocity/sample size

for the 10 shot string, N=10, mean=3707.20, ES=65, Variance=313.56, SD=17.71, then at a 95% confidence level I gets a maximum 3720.55 average fps high and a minimum 3693.85 fps low, or I could expect my velocities to be in that range at a 95% confidence level. T Val 2.26, N-1

for the 5 shot string, N=5, mean 3699.80, ES=40, Variance=77.36, SD=8.80, then at a 95% confidence level I gets a max 3712.01 average fps high and a minimum 3687.59 fps low, or I could expect my velocities to be in that range at a 95% confidence level. T Val 2.78 N-1

I played around with a 20 shot string that had the same ES as the 10 shot one and the results were very close in all aspects.

This would indicate if you mess with stuff like this go for a 10 shot or 10 sample. The 5 shot string having a ES of 40, some 25 fps less than the 10 shot string resulted in a greater velocity range. The term, SD or standard deviation is tossed around frequently and implies a normal distribution or that bell shaped curve.

Then how much trajectory differences be realized with these minor differences- not much for my casual varmint shooting.