All things considered, how much emphasis do you put on standard deviation in final load selection? Is it a major factor in final selection of a load? I understand lower is better, but what is a good realistic SD?

SD is not very important at all in most cases. If you are shooting beyond 500 yards with a 1/2 MOA firearm, then it becomes something to consider. The other case it might be important is if it is seriously higher than normal. That might also indicate a loading error.

The commercial ammo I have checked (not many cases) tended to have about a 35 FPS standard deviation. The worst was some of the special high energy stuff, which was about 65 FPS.

My rifle handloads tend to be around 25 FPS or so. Handgun loads tend to be in the single digits or low double digits. I got on a kick once, and loaded some 223 that was in the single digits. I couldn't see any difference between that and the regular stuff.

Another comment is that most handloaders calculate the SD of far too few shots to be meaningful, often just because there's a SD function on their chronograph. The ammo factories calculate SD from several dozen shots, and sometimes even more for match ammo, not three or five or even 10. It's often illuminating to calculate the SD of two separate 5-shot strings of the same load. The difference can be considerable!

Another factor is that most of the "affordable" chronographs handloaders use have too little screen spacing to provide consistent velocities for individual shots, so any SD calculations are just about meaningless. The average for 10 shots may be very close to the average from chronograph with a much longer screen spacing, but the individual shots will differ considerably. I know this partly from lining up more than one small chronograph with my Oehler 35P and recording the velocity of each shot from both chronographs.

MD is absolutely correct on the notion that a sample of five or so is much too small to give precise estimates of long term SD. A 1" rifle will routinely print 1/2" to 1 1/2" groups with no change whatever in ammunition, technique, or the firearm. Five piece samples from a large population of ammunition with an SD of 25 will routinely produce SD estimates between about 15 and 75.

I'm not so sure about the notion that the uncertainty in screen spacing affects SD. I think that is more likely a constant error, which will not affect SD. Otherwise, I think it would not be possible to get SD readings in the single digits.

Ditto to what Mule Deer and Denton posted about sample size, etc. Just because a device will calculate a statistic doesn't mean it's meaningful.

OTOH, there 's an old dude (older than me, even) who frequents our range and fires hundreds of rounds, every one over a chronograph, and meticulously logs all of them. He scoffs at SD, doesn't even know what it is.

He has fun, though.

Paul

To give you an idea, SD is normally considered to not get any more accurate once 1500 samples have been taken. So five or ten is not really getting started even.

This is a major reason polling is so fickle in elections.

Not only is SD not significant in the low sampling strings often used but it shouldn't even be considered. It is not representative. I come from a heavy stats and metrology background and the willy nilly use and faith of chronographs bother me. Great tool, indispensable in fact but the level of precision people quote from their "not calibrated", unreported uncertainty of a device is a joke. in fairness this has a lot to do with how people report data. Just a snob and my turkey isn't done yet

smallfry....

Where in Idaho are you? I grew up in Rigby.

All replies to my OP seem to be the stuff that needs to be better understood from technical side instead of the "what I've always been told" side. I have known hunter / shooters with decades more experience than mine that put a great deal of stock in this data. My own experiences tend to not factor it into the equation as much as I have been led to believe. I appreciate the feedback. I have a new MagnaSpeed chrony which I think is a step up from my Pro Chrony, but velocity is my most useful function of the device. Its definitely more consistent and has yet to miss a shot and don't give a hoot bout sunny / overcast conditions. Thanks to all who contributed.

CDA! You still consulting?

Why worry about so? Let the group size on the target tell you. Just because it is easy to calculate doesn't mean it tells you anything.

I think Muledeer's point about chronograph shays to do with the fact that the shorter the distance between the screens the more likely that small measurement errors will be multiplied. That's why the Oehler 43 has three sensors and compares the reading from 1-2 to 2-3. Don't they cal that the proof channel? Hard to remember after a mound of turkey, several fingers of brandy, and the packer to under review!

30 samples is generally considered a statistically valid number for calculating a reliable SD.........but obviously, more is always more reliable.

MM

Yep. When I can find someone who will hire me.

If you are talking 100 yard or 200 yard groups and are trying to build a load to shoot much longer distances, the group size won't necessarily tell the whole tale. A load with a high extreme velocity spread (or standard deviation if you chrono enough rounds to be valid as others have said) will start to exhibit vertical stringing at longer distances. That is not good. You already have to deal with estimating wind drift, throw in vertical dispersion on top of that and your probability of a first round hit drops even more.

For a rifle I don't plan on taking any shots over 4-500 yards with, I pay little attention to ES or SD if my 100 or 200 yd groups are small, so in that regard I agree with forpest. Just my 2 cents and worth every penny.

A ten-shot standard deviation is significant. About like a single three-shot group.

Sambo,
Agree to a certain extent. Many of my friends and I try real hard to get our extreme spreads under double digits and succeed in some caes but only with equipment most guys won't spend the $$ on.
I don't think there is any way with a mechanical scale or most strain gauge electronic scales that a loader will succeed at doing it. The rest of the detail including weighing brass, reducing concentricity to under .003", micrometer bullet seaters with the proper stems to suit the bullets being used, match primers, primer pockets uniformed, and annealed necks every second firing have to be paid attention too.
It help too if whatever caliber a guy is shooting Lapua makes brass for it. Shoulder bump and neck clearance under .003" helps to eliminate any doubt there too. Bottom line as many of us know it is a pain and a lot of work to get there however I find it well worth the effort and that's what makes what we do fun. I have to know where the clean bore - cold bore shot is going to impact and to be confident ES needs to be as low as a guy can possibly get it IMHO.

Dave