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.
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, 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?
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.
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. [img]images/icons/grin.gif[/img]
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,
Drop at 600 yards,
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. [img]images/icons/smile.gif[/img]
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. [img]images/icons/grin.gif[/img]
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.
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...