Chronograph question.

[Brent]

Penn,

SD is a measurment of uniformity, I believe concerning velocity, the SD is the number 65% of your shots MV will theoretically fall within, hence the smaller number the better. This doesn't mean to worship SD, just "consider" it. It doesn't mean you will find good groups when you find the lowest SD, wouldn't that be nice tho...

If you find good 10 shot groups with a load that has an acceptable ES, say 10-15 fps and SD is real low, say 3-5, chances are the ES is high because of a just a couple of shots at the extreme edge.

The higher the number of shots, the more accurate the SD numbers will be.

To tell you the truth, I pay less attention to SD and more to ES than anything.

If you had an SD of 47, the ES was totally unacceptable in my opinion. On the other hand, if ES was 47 and SD was 7 I'd see why ES was so high.

I'd mark the case or two that caused the high ES and repeat the test to see if the played along or were causing the problem, cull them if they were. Even if they fell into the group well at 100-200yds, they're going to kill you with fliers at LR if they cause that high of ES.

 

[PAMONTY]

I understand what average velocity is, I understand what extreme spread is, but I DON'T understand what standard deviation is.
I was down at the range this morning and shoot a few groups. Best standard deviation I got was 7 out of 5 strings fired. Worst was 47 out of those same strings.
I also noticed that best S.D. was not the tightest group.
Could anyone explain the connection?

By the way the chrony is a ProChrno digital.
Thanks

 

[PAMONTY]

Brent
Thanks, I have mainly used my chrony for velocity readings in the past. I can see now how to find bad brass in the lot.
Funny you picked up on the 47 SD, this string had a ES of 104 fps. I was shocked at first but after looking into things closer I found A Winchester case in with my Norma. All the other brass was Norma. I would have never guessed brass would make that much differance.
Also in the past I have tried to keep my ES under 30. I'm getting the feel that this maybe pushing the outside edge. I'm going to do some tests and see if I can get this number to tighten up.

What would be a good number of rounds to fire to get a accurate reading?

 

[Josh Amerine]

Dear Penn:
The standard deviation is a calculation of the distance each data point (velocity of each round) falls from the central tendency (mean, median or mode). The standard deviation is one of the ways of measuring variance. It is a MUCH more meaningful indication of variability than extreme spread because it uses each one of the data points in the calculation.

Hope this helps,

T Sharps

 

[Brent]

Penn,

You should be able to get that number to tighten up better, getting it down to below 30 is usually fairly easy, into the 10-15 range can be a bit more time consuming but, that would take all the fun out if it wasn't.

There's a lot of things that can be causing a high ES, in your case this time it was the odd piece of brass, but I'd caution anyone to mark and retest a piece of brass (same lot#) before they just cull it on the first wild velocity it's associated with, there's a very good chance it's ignition, neck tension or something else as well.

Statistitions, I believe, say any amount tested less than 30 isn't giving you the real story. This might be true, and maybe we should all shoot thirty shot groups too... The reality of it is, the more the better, and ten shots is plenty good for me at this point. I'm always comparing them to the other ten shot strings so it's relative and I think that tells me more than anything if changing components etc while looking for smaller numbers.

A side note on what's acceptable at what range...

Example; 178gr A-Max I been shooting in my 300 Ultra at 3230 fps.

Drop at 1000 yards,

-239.9" @3230
-242.7" @3215
-245.6" @3200


Drop at 600 yards,

-59.5" @3230
-60.3" @3215
-61.1" @3200

So, if that gun shot perfect one hole groups at 100 yards, it will never consistantly shoot better than 1.6" groups at 600 yards or 5.7" groups at 1000yds. Here enters the problem tho, most guns shoot a tad more than perfect one hole groups at 100yds to begin with, so the additional departure angle combined with the MV variation can open groups beyond those figures easily. How much? What MOA is the gun consistantly shooting at 100 yards? Times that number by ten then add it to the MV variation in inches and that's likely the best it'll ever do with that load...

What might be totally acceptable at say 600yds, may open up groups at 1000 yards enough to keep from "guaranteeing" a kill zone hit, depending on the animal size... something to decide based on max range you might encounter the game, and how far you're confident in your shot placement ability yet...

Hope that helps some.

 

[Josh Amerine]

Ian, you are close to right. Just say that standard deviation is a measure of uniformity relative to the mean (average velocity).

T Sharps

 

[Brent]

Jerry,

we have seen "round" groups with different vel readings. What I mean is that a "slow" vel shoots higher then a "fast" one, or to the left or right. We have forgotten that vel also relates to barrel time and its harmonics, which in turn determines what barrel location the bullet sees as it leaves the muzzle.

There are so many variables in shooting that vel alone cannot determine accuracy. However, if I had two equally shooting loads, I would choose the one with the lower vel variations..just in case.

I've seen the same thing time and time again, that's why I only shoot groups, for the average and not just a shot or two at each range. The last paragraph of yours here too is my exact sentiments.

I also try and do all my load development at 200yds or beyond, I think my Dad's finally convinced of that too, it took a while.

If wind is a real big issue, I try to keep her back to 200, if not, it's out farther until I feel the wind might have an affect that skews the results any... the farther the better IMHO.

Ian,

Low SD's and smaller groups... I'm right with you on that!

ES "can" always screw the shot in enough distance, especially if the shot leaves high for group to begin with and the MV was higher than the mean. If the shot leaves low for group, it can cancel out the effect of a higher than the mean MV, maybe all the way to 1000 yards if it left low enough, or vice-versa...

SD basically tells me what range I'm likely to see the majority of my shot fall into.

AVE tells me where they'll likely be centered around.

I've got it around here somewhere but can't find it anywhere so, can anyone refresh my memory as to how to figure the SD for a ten shot string? I believe it was in an old Shooting Chrony manual of mine I can't find.

I'm not sure SD is a referance to the average MV number or not, that's why I'd like the formula. I don't think it is relative but I may be wrong...

 

[PAMONTY]

Well I really got in over my head with that question, Lots more Info then I expected. I do understand it a little better, seems to me bottom line is lower the numbers the better. The rifle I was shooting is a 243, rebarreled with a Shilen tube #5 contour. I use it mainly for deer hunting.
On the other hand I have a 6.5x284 I use for most of my long range ground hog hunting. This is the one I try to do my very best with. I prep brass pretty much the same way with all my guns.

Deburr inside flash hole.
Clean primer pocket with a tool that sets all primers at the same depth. [name??]
Turn outside neck.
Measure all charges with digital scale.
Resize with neck bushings, to get.0001 press

I'm not sure what else there is to do. I haven't checked the 6.5 lately so not sure what the ES-SD is. Thinking 20-25 for ES.

I have to say though, if I had to point my finger at one thing that accounted for most of my misses it wouldn't be the equipment. It would be misjudgeing yardage. Lazer rangefinders leave alot to be desired sometimes.

 

[Josh Amerine]

Dear Brent:
Here is how you calculate standard deviation.

1) Calculate the mean (average the velocity of the shots...add up all the velocity scores and divide by the number of shots).

2) Determine the amount that each shot deviates from the average. (ex. average velocity 3000, shot #1 is 2990, deviation from mean is -10)

3) Square each deviation score. From the above example -10 x -10 = 100.

4) Add all the squared deviation scores together.

5) Take the square root of that sum of squares (what you did in step #4).

That is the standard deviation (from the mean or average).

Hope this helps but if you want a two-way ANOVA, we are not doing it on-line.

T Sharps

 

[Mysticplayer]

Brent, excellent observation, however, we have seen "round" groups with different vel readings. What I mean is that a "slow" vel shoots higher then a "fast" one, or to the left or right. We have forgotten that vel also relates to barrel time and its harmonics, which in turn determines what barrel location the bullet sees as it leaves the muzzle.

There are so many variables in shooting that vel alone cannot determine accuracy. However, if I had two equally shooting loads, I would choose the one with the lower vel variations..just in case.

Ultimately, the only way to know how a load performs is to shoot is at the range you want to use it at. Like Brent, I have personally had several one hole loads at 100yds that wouldn't stay on a piece of paper at 500yds.

Now, I work up my loads at 180 or 250yds. I find this to be a better representative of LR performance. In general, if a load works well at these ranges, it will work well at further ranges. I find wind and conditions to play a bigger roll at that point.

Jerry

 

[Ian M]

Brent,
Since I am hopeless with numbers and stats etc. I ******** the troops and simply say that the SD is an indicator or how uniform the reloads are, relative to each other. Sounds impressive, guys always nod their heads as if I must know what I just said and carry-on. If low SD's meant smaller groups the world would be a simpler place to live in.

 

[jb1000br]

Penn Monty - SD in my mind is most easily described as:

the average deviation from the average

dont know if this helps your understanding, but that's the way i think of it.
JB

 

[bblaine2k]

I think that T-Sharps must have some background in stats, science or engineering. Not that that's a bad thing, just takes one to know one! Just an observation from one number cruncher to another

 

[Josh Amerine]

Mountaineer, is it that obvious? I thought I was being SO clear and easy to understand.

T Sharps