SD's and ES's??????? Help!!

Elkslayer1

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Jan 22, 2010
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68
Ok,

I hear alot of talk about Ds's and Es's. I have a chronograph but use it just to get the speed of my best round to order turrets.

Can you geys give me the ins and outs of Sd's and Es's.

Ps....most of my guns shoot MOA out to 700 yards.

Thanks!
 
It's ES or extreme spread that makes the difference. ES is the difference between the maximum velocity in a particular group of shots vs the minumum velocity in the same group of shots. Too much ES and you will get vetical stringing of your groups at long distance. Too much ES will also make the wind affect your shots differently at the same distance.

Let's say you're shooting at 900 yds. you've got a 15 mph wind blowing from 04:30. and your loads have got an ES of 75 feet per second. (We'll assume you're shooting a 6 Long Dasher and a .48 BC bullet, cause I've got that loaded into my program)

If you happen to shoot the extreme ends of your velocity for your first two shots (entirely possible) then the first one would impact over 8" different vertical and 2" different horizontal from the 2nd shot............just because the difference in the velocity of round 1 vs round 2. This is ONLY taking into account the delta V, not cold bore vs warm bore or normal variation in point of impact due to alot of different things.

Now, lets take the same situation and use an ES of 20 feet per second. Your difference in point of impact for the same two shots would be only 2" vertical and 1/2" horizontal.

SD or Standard Deviation is much harder to explain and calculate. It is basically a Statisticians argument and takes alot of shots over a very accurate chronograph to accurately determine. I wouldn't worry about it.

If you only plan on shooting out to 700 yds, then the differences quoted above would be much much less. If your 1 moa groups at 700 yds are nice and round, and don't have more vertical than horizontal........you're ES's are probably ok.............just depends on how far you're gonna shoot and what size targets you plan on shooting at.

Hope this answers your Q's.
 
SBruce thanks for your reply!!!!

How many shots would you consider a good number?

What are good E's ranges for 7MM and 30 Cal rifles?

Thanks
 
SD or Standard Deviation is much harder to explain and calculate. It is basically a Statisticians argument and takes alot of shots over a very accurate chronograph to accurately determine.

Most all can readily understand average MV and Extreme Spreads in measured muzzle velocity. But how does one explain the uniformity or the dispersion of MV? If the ES is 50fps, how many bullets out of 100 fired leave the muzzle with velocity that varies by 10 fps, 20 fps, 30 fps, or 40 fps?

That's what Standard Deviation (SD) provides. It's a measure of the uniformity of your MVs. Most chronographs will calculate SD these days. If you have an ES of 50, you don't know how many bullets out of 100 fired will leave the muzzle with that difference of 50 fps. If you shoot over your chronograph during load development, by the time you're done, you'll probably have enough rounds fired to determine a reasonably correct SD. An SD of 15 means that 2/3's of your bullets will leave the muzzle within 15 fps of your average (mean) MV.

Example using this data set:
Average MV = 3000 fps. ES = 30 fps. SD = 10 fps.
The SD of 10 means that 2/3 of your shots should be expected to have a MV between 2990 and 3010 fps.
Which also means that 1/6th of your shots should be expected to have a muzzle velocity between 2895 and 2990 fps.
And the remaining 1/6 of your shots should be expected to have a muzzle velocity between 3010 and 3015 fps.

I can usually get SDs down to 15 fps or less with some load development time and my goal is an SD of 10 or less. Any SD of less than 10 fps is pretty darn good for a large capacity rifle cartridge like the 7mm Rem Mag or 300 Win Mag. The smaller the better - same as with group measurements. However many consider any single digit SD to indicate a very acceptably uniform load with 7mm Rem Mag and 300 Win Mag class cartridges for purposes of shooting long range (~1000 yds).
 
Most all can readily understand average MV and Extreme Spreads in measured muzzle velocity. But how does one explain the uniformity or the dispersion of MV? If the ES is 50fps, how many bullets out of 100 fired leave the muzzle with velocity that varies by 10 fps, 20 fps, 30 fps, or 40 fps?

That's what Standard Deviation (SD) provides. It's a measure of the uniformity of your MVs. Most chronographs will calculate SD these days. If you have an ES of 50, you don't know how many bullets out of 100 fired will leave the muzzle with that difference of 50 fps. If you shoot over your chronograph during load development, by the time you're done, you'll probably have enough rounds fired to determine a reasonably correct SD. An SD of 15 means that 2/3's of your bullets will leave the muzzle within 15 fps of your average (mean) MV.

Example using this data set:
Average MV = 3000 fps. ES = 30 fps. SD = 10 fps.
The SD of 10 means that 2/3 of your shots should be expected to have a MV between 2990 and 3010 fps.
Which also means that 1/6th of your shots should be expected to have a muzzle velocity between 2895 and 2990 fps.
And the remaining 1/6 of your shots should be expected to have a muzzle velocity between 3010 and 3015 fps.

I can usually get SDs down to 15 fps or less with some load development time and my goal is an SD of 10 or less. Any SD of less than 10 fps is pretty darn good for a large capacity rifle cartridge like the 7mm Rem Mag or 300 Win Mag. The smaller the better - same as with group measurements. However many consider any single digit SD to indicate a very acceptably uniform load with 7mm Rem Mag and 300 Win Mag class cartridges for purposes of shooting long range (~1000 yds).


+1

Very well said.

The lower the number the the more consistant the rifle/load is shot to shot.

J E CUSTOM
 
Phorwath,

Thanks for the info. That clears up the SD's some and makes alot more sense than the prior explanations that I'd heard!

How many shots does it take to get a reliable and "real" SD?? is it 100??

Is a shooting chrony a good enough instrument to get a true SD? Or does it take alot more expensive (and accurate) chronograph, like an Ohler 35P or similar?..........Garbage in, Garbage out type of thing.

Some will argue to the death saying that a 3 or 5 shot group isn't enough to really determine average group size. The same argument (I am assuming) would apply to the SD's given by the chronograph if only 10 or 20 shots are fired through one??

If I measure the length of 20 fence posts out of a pile of 200 and figure the SD based only on those 20, wouldn't it be entirely possible to be an incorrect value.?
 
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Phorwath,
My answers are conditioned on the disclaimer - IMO

How many shots does it take to get a reliable and "real" SD?? is it 100??
Approximately 20-25 shots. Having stated that, I want to make it clear that I don't proceed to shoot 20-25 shot groups for purposes of collecting Avg MV, ES, and SD in one setting (one firing session). Because that's not how I develop, identify, and field proof my pet handloads. I find that by shooting over my chronographs during load development, after I've settled on a load, and then while field-proofing the accuracy and field drops of that load, that I often end up with 20 or more shots fired over the chronograph. I log atmospheric temperatures every time I shoot, because temperature is one known factor affecting MV. Doesn't do any good to shoot 5 groups of 5 at 0F, 20F, 40F, 60F, and 80F, unless you've already demonstrated that the powder and primer don't yield differing MV at the differing temperatures. I seldom shoot more than three-shot groups for accuracy. But I don't shoot one single 3-shot group to establish accuracy. I shoot a number of 3-shot groups over time at different ranges. I will, on rare occasions, shoot more than 3-shot groups solely in order to collect additional MV data, but generally only AFTER I've selected/finalized my load and I want to field proof the Avg MV, ES, and SD.

Is a shooting chrony a good enough instrument to get a true SD? Or does it take alot more expensive (and accurate) chronograph, like an Ohler 35P or similar?..........Garbage in, Garbage out type of thing.
I've never owned a Shooting Chrony, but based on numerous posts I've read regarding user experiences with the Shooting Chrony, I think not. One LRH member posted that the only thing his Shooting Chrony was good for was to confirm a bullet left the muzzle and passed over the skyscreens. Another posted that the best shot he fired over his Shooting Chrony was the one he killed it with. Without pumping any specific brand of chronograph, I will say that the chronograph must produce accurate data to be useful for the collection of any beneficial data - whether that be Avg MV, ES, or SD. If the chronograph data aren't accurate, the chronograph not only provides misleading SDs - it also provides inaccurate Avg MV and ES. Here's a link to some photos, and a description of, my current chronograph setup:
http://www.longrangehunting.com/forums/f19/new-oehler-35p-new-skyscreen-rail-first-test-data-60778/


Some will argue to the death saying that a 3 or 5 shot group isn't enough to really determine average group size. The same argument (I am assuming) would apply to the SD's given by the chronograph if only 10 or 20 shots are fired through one??
I would agree that a single 3 or 5 shot group is insufficient to determine the average accuracy (group size) of any rifle. But five 3-shot groups fired over a period of days while field proofing the rifles capability will begin to more closely represent the true average accuracy of the rifle. Ten 3- shot groups will add an even higher level of confidence of the rifle's true accuracy capability. Same with the determination of Avg MV, ES, and SD. 20 shots with identically loaded cartridges fired at similar temperatures will provide a high quality determination of SD. I shoot thinner tubed barrels than most of the members on this Forum because my hunting is backpacking in 4-10 miles and then back out - hopefully with some wild game meat. I see my MVs change some on my thinner tubed barrels with increasing barrel temperatures. They tend to display a decreasing trend in MV as they warm up. I don't ever read anyone else posting the same thing with the heavier tubed LR rifles in common use. But I observe this often enough to believe that my warming barrel increases my ES and SD. This is another reason I fire 3-shot groups rather than 5 (or more) shot groups. Again, I don't shoot 20 shots in one sitting solely for the purpose of collecting 20 MVs. I collect these MVs over time as I'm developing and then field-proofing my pet handloads and confirming drop data at variable distances. By the time I've fleshed out my chosen load, I find that I usually have collected enough MV data to establish a very representative Avg MV, a realistic ES, and a realistic SD.

If I measure the length of 20 fence posts out of a pile of 200 and figure the SD based only on those 20, wouldn't it be entirely possible to be an incorrect value?
Yes, it would be possible. Even probable - without knowing something about the methods of fabrication of the fence posts. Like were they all intended to be fabricated to the same length? Some due diligence is required in the selection of those 20 posts from the 200 posts, and the measurements need to be carefully collected and recorded. Just like the loads being fired over the chronograph need to be hand crafted carefully and fired at similar ambient temperatures which most closely resemble the first shot or two from the fouled barrel. MVs can be vastly different from a squeaky clean bore, when compared to a fouled bore. I've had clean bore velocities reduced as much as 120 fps compared to the second fouled bore shots from two of my rifles. Generally speaking, collection of MVs over the chronograph requires some thought and attention to factors that could bias the MV data high or low, compared to the conditions under which you'll be taking the shot at your game animal. The collection of MV data should attempt to duplicate the conditions under which the rifle will be fired during the hunt.

Back to the fence posts; if the same person cut all 200 fence posts using the same equipment and methods, and you selected your 20 fence posts for measurement by collecting one from every 10th fence post cut, and then carefully measured and recorded the length of these 20 fence posts - your Avg fence post length, Extreme variation in fence post length, and SD from the Avg fence post length should all be pretty close to the reality of what's likely to be present in the remaining 180 fence posts.

The fence post example is not a very good analogy to the collection of MV data. But I tried to identify some pertinent factors that should be considered with the collection of the fence post length measurements. There are also factors to consider with the collection of MV data, if a shooter hopes to establish Avg MV, ES, and SD that are representative of the true Avg MV, true ES, and true SD likely to be in-play when pulling the trigger on the round that's intended to slay the prey.

FYI: I am not a statistician. I'm sharing some thoughts and my opinions based on my knowledge, and experiences. Good shooting to ya.
 
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SBruce thanks for your reply!!!!

How many shots would you consider a good number?

What are good E's ranges for 7MM and 30 Cal rifles?

Thanks

You're Welcome, but I really can't answer about 7mm and 30's because so far, all of my long range and benchrest experience is with 22's, 6mm's and 25 calibers.

Kirby Allen is building me a 300 WSM as we speak though.:D

With the smaller calibers and way way lower BC's, I've always tried to keep the ES under 30 fps (not often successfully), measured over at least 10 rounds, but usually more like 30 rounds.......I figured that ES under 30 automatically and generally means a low SD also (I never had a chrony that would calculate SD, so I had to do it manually if I wanted that info)

However, if I was getting good groups at extended ranges, I never even bothered with it; I just shot the guns at the ranges I planned on using them at, recorded POI and conditions, and had fun. I figured that 1/2 moa under good conditions, off the bench was plenty good for hunting purposes........better than I can shoot under hunting/field positions and conditions for sure, and I knew for a fact where they were shooting under different temps/conditions that way too.

Phorwath had given a great explanation of SD's however, and I am assuming that has answered your original Q's??
 
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