Es/sd

[Greywolf18]

Sorry for the newbie question but what is ES & SD? I see posting about it for load development but don't know what they are talking about. How does one test for them? Thanks for the info!

 

[Pdvdh]

ES = extreme spread of muzzle velocity. The highest muzzle velocity minus the lowest muzzle velocity.

SD = standard deviation of a set of muzzle velocities. This describes the expected spread of muzzle velocity - above or below the true mean of muzzle velocity based on a set of recorded muzzle velocities. You could Google search the term and get the exact definition. It's used to analyze data sets in Statistics.

Muzzle velocities are typically obtained with chronographs, which typically cost somewhere between $125 and $400, depending on the Brand and Model.

 

[MontanaRifleman]

ES = extreme spread = the spread between the highest and lowest velocity in a group.

SD = standard deviation.

A defintion of SD from wiki...

shows how much variation there is from the "average" (mean). A low standard deviation indicates that the data points tend to be very close to the mean, whereas high standard deviation indicates that the data are spread out over a large range of values.

Standard deviation - Wikipedia, the free encyclopedia

These numbers are derived by shooting through a chrony that measures bullet velocity. I personally dont put much stock in them because chrony's are not precision instrumnets. I have seen a lot of inconsistancies with my chronys (I have two) When I shoot them back to back their velocites will vary anywhere from 5-60 fps.

If you shoot a 5 shot string and get the following velocities...

3056, 3067, 3094, 3094 and 3069, Your ES is 3094 - 3056 = 38

However, the odds are very lilkely that your actual ES is smaller.

The reason being, Your chrony is accurate to plus or minus "x" fps. The shot that measured 3056 might actually be 3070 and the shot that measured 3094 might actually be 3080. For this reason, I put almost no stock in chrony measured ES's. I use actual group results to determine my best loads.

A good experiment to try is go out and shoot a 5 shot string, record the results and repeat the next day under similar light conditions (light conditions will affect readings) I can pretty much guarantee you that you will get different results.

 

[Pdvdh]

If you shoot a 5 shot string and get the following velocities...

3056, 3067, 3094, 3094 and 3069, Your ES is 3094 - 3056 = 38

However, the odds are very lilkely that your actual ES is smaller.

The reason being, Your chrony is accurate to plus or minus "x" fps. The shot that measured 3056 might actually be 3070 and the shot that measured 3094 might actually be 3080. For this reason, I put almost no stock in chrony measured ES's. I use actual group results to determine my best loads.

Mark,
So even when the recorded velocities are collected in the same lighting conditions within a 20 minute period of time, you place no confidence in the ES?

I could understand not trusting the accuracy of the numbers; for example, is it really 3057 fps or is the true velocity 3068 fps. But I don't understand the dismissal of the variance (or spread) of the velocities on a given shot string. When I run two chronys in tandem and for five shots, the difference between the recorded velocities on the two different chronys for each shot is 15, 15, 16, 18, 16 fps, then I conclude with confidence that I've closely established the spread between the highest and lowest recorded muzzle velocity for those 5 shots.

Particularly when I receive the same type of delta between recorded velocities day after day after day over the dual chronograph setup.

I believe there is validity to the conclusion that the accuracy of the recorded velocity can vary under different lighting conditions; that a bullet whose true muzzle velocity is 3000 fps might be recorded as 3020 fps in one lighting condition and 2980 fps under a different lighting condition. But when two different chronographs spit out muzzle velocity differences within a 4 fps window, shot after shot in the same lighting conditions, I have to reach a different conclusion regarding the reality and accuracy of the ES.

I place high confidence in the variance in velocities if my deltas over the two chronos are consistently within 4 fps. I accept the fact that the accuracy of the recorded mean velocity of a particular load from day to day under differing lighting conditions may be off a bit either high or low. The MV I input in my ballistics program is the mean of the means collected from separate data sets for that same load, with some consideration given to the different ambient temperatures for each set of data, since powder burn rates and muzzle velocity would be expected to vary under differing ambient temperatures.

Lastly, and only after I've selected my final load for a rifle, I like to collect velocity over the chronographs as far downrange as possible. Farthest I've collected the velocity data to date is 990 yds. Then I tweak the BC in my ballistic program until the predicted 990 yd velocity matches the 990 yd chrono'd velocity and see how the actual measured drops compare to the predicted drops. So far they match very well.

That's how I use and apply the data from my chronographs - for better or for worse.

 

[MontanaRifleman]

Paul,

First I should mention that in my list of made up velocities, I didn't intend to type 3094 twice. No big deal.

If I was consistently getting differences of only 1-4 fps, then I would put a lot more trust in my chronies. But I am getting much bigger differences... 5-60 fps, set up back to back (and set up carefully). There is no way that I can come to any solid conclusion about the ES that these chronys are telling me. One might give me an ES of 10 and the other an ES of 60 with the same shot string.

For me, the bottom line is the actual results downrange that count.

 

[Pdvdh]

10-4. With those MV inconsistencies, I understand.

Down-range groups and drops are the final say-so for sure. I use my chrono data as an aid to getting there - sometimes with less time and effort than without. I also like the down-range velocity confirmation. Combined with good correlation to drops, it helps ensure the ballistic software inputs and program are providing realistic predicted data at other ranges and under other atmospheric conditions.

 

[LouBoyd]

10-4. With those MV inconsistencies, I understand.

Down-range groups and drops are the final say-so for sure. I use my chrono data as an aid to getting there - sometimes with less time and effort than without. I also like the down-range velocity confirmation. Combined with good correlation to drops, it helps ensure the ballistic software inputs and program are providing realistic predicted data at other ranges and under other atmospheric conditions.

If you measure the ES while shooting at 100 yards certainly the group at 100 yards tells you more about the accuracy of the load at 100 yards. But I'd expect the ES to be a better predictor of what the groups would be like at 1000 yards or more than what the group size at 100 yards tells you. That's assuming that neither the rifle or the chronograph have a severe defect.

Bullet drop is proportional of the square of the time of flight so small velocity variations cause a much larger percentage of the group size at long range (many hundreds of yards) than they do at one or two hundred yards.

Wind deflection is proportional to the actual time of fight minus the "vacuum time of flight". (muzzle velocity times distance). Velocity variations do affect wind deflection but only as it directly changes those real and potential times of flight. There is no time squared term related to wind deflection calculation. Generally wind deflection increases over a given distance with decreasing muzzle velocity but there is a dramatic reversal of that trend just below the speed of sound.

Wind deflection uncertainty is determined by the shooters ability to judge the wind over the path to the target and to apply that to it's effect on the bullet trajectory. Velocity uncertainty is related the quality and consistency of the loads, and also the condition of the bore from shot to shot. Either wind estimation uncertainty or vertical stringing can be the limiting factor on group size. The accuracy of a marksman (or rifle) at 100 to 200 yards can be nearly unrelated to their accuracy at 1000 plus yards since the dominating errors may be from completely different causes.

 

[MontanaRifleman]

The first question is... is your chrony accurate? If you think so, how do you know? Most chrony owners can not answer those questions. Even if you knew for certain that your chrony was accurate, a low ES does not necessarily make an accurate load.

The farthwer down range you can shoot for results, the better. I look mostly at verticle dispersion when evaluating a load.

 

[Pdvdh]

The first question is... is your chrony accurate? If you think so, how do you know? Most chrony owners can not answer those questions. Even if you knew for certain that your chrony was accurate, a low ES does not necessarily make an accurate load. Correct, but a low ES reduces vertical stringing at long range and if I get low ES and good accuracy at 300 yds, I don't have to be concerned about vertical stringing at 1000 yds. I have great odds that load will be good to go at 1000yds.

The farther down range you can shoot for results, the better. I look mostly at verticle dispersion when evaluating a load.

I know of no way to absolutely prove a chrony is spitting out exactly correct velocity. This is somewhat akin to the girlfriend asking for proof that you love her. If you can't see it, it's tough to prove it. Calculating the average of two recorded velocities by shooting over two separate chronographs that have been demonstrated to operate reliably would be a step towards establishing greater assurance of an accurate, true MV. Adding a third quality chronograph would add even greater confidence. If I fired over 10 separate chronographs, tossed out the lowest and the highest velocities, and took the mean of the velocities from the remaining 8 chronographs? Now I think we'd be getting pretty darn close to the true velocity. But who wants to buy, operate, and maintain 10 chronographs? Sooooooooooooooo....

Here's how I go about improving the accuracy, credibility, and confidence of my chrony data. Using two chronys in tandem is the first step toward more credible chrony data, as stated in my earlier post in this thread, and as I have in other posts/threads in the past. So I run two chronys in tandem. Two chronys is the bare minimum required in order to be able to establish that the instruments are spitting out credible numbers. It would appear that at least one of your two chronographs isn't performing reliably, if I understood your earlier post correctly.

With the two chronys recording each shot, the next step is demonstrating that the delta between the differences in recorded velocities from the two chronographs is in the single digit range - the lower the better. By this I don't mean that the difference between the velocities reported on one chronograph for each shot string is 9 fps or less. A delta of 5 fps or less for the differences of the recorded velocities for 4 separate shots means that if the one chrony reads 3005, 3010, 3017, and 3008 fps then the second chrony reads 3005 to 3010, 3010 to 3015, 3017 to 3022, and 3008 to 3013 fps for those 4 shots, respectively. The ES from the first chrony is 12 fps, however the delta between the differences in velocities recorded by the chronys is 5 fps or less. After establishing the 'normal' delta for the chronys over repetitive shootings, we have established a means of identifying bad velocity data. If I fired a 5th shot and my chronys recorded velocities of 2981 and 3012 fps, this 31 fps difference is an obvious red flag, because I normally obtain a delta of 5 or less. Clearly, one or both chronographs hiccuped on this fifth shot and spit out a bad velocity. The 2981 fps is likely the bogus one because it's so far outside the mean of the first four shots, while the second reading is right in line with the first four shots. With consistently repetitive single digit deltas (with my chronys, generally less than 5 fps), I have a method to identify faulty velocity data from any individual shot fired. I discard these bad recorded velocities. I don't include them in any data sets when determining MV, ES, or SD. Last weekend I fired 8 shots over my chronographs and the delta was within 4 fps for that 8-shot string.

At this point in the process, I've demonstrated my chronys are operating reliably. I also obtain two readings for each shot. Each of these two measures improve the credibility of my recorded velocities compared to operating with a single chronograph.

Next step is to work up loads while recording velocity data. Loads with high ES get eliminated even if they produce good accuracy. Loads with low ES and good accuracy get further attention. I develop loads by shooting at 300 yds. If I get good accuracy and repeatable low ES at 300 yds, that load is generally going to be a winner at farther ranges.

After I've honed in on the load I intend to use, I shoot for groups at 1000 yds, recording group size and drops. If the load's still a winner at 1000 yds, then with the known 1000 yd POI, I set up my chronographs at 1000 yds and obtain 1000 yd velocity.

Can I prove my MVs or 1000 yd recorded velocities are accurate? No, not with absolute certainty.

With the MV, the 1000 yd velocity, and known atmospheric conditions, I can calculate an accurate BC for my bullet. If the chronys read high at the muzzle, they'll read high at 1000 yds also. If they record velocity slightly lower than the actual (true) velocity at the muzzle, I fully expect the 1000 yd recorded velocities will be equally low. The difference in the velocities at the two ranges is what's important for BC determination. If my recorded velocities are 20 fps high at the muzzle and at 1000 yds, or 25 fps low at the muzzle and at 1000 yds, I'll still get a very accurate BC detemination from the velocity data for my bullet. An important (to me) advantage to collecting the 1000 yd velocities is that the affect of any error in the measured velocities or the measured distance is diminished in the calculation of BC. There is such a great loss of bullet velocity that an error in recorded velocity of 20 fps has relatively little affect on the accuracy of the BC calculated over that large velocity loss.

I also use the chrono'd MV, 1000 yd velocity, and the 1000 yd drops to calibrate and verify my ballistic program. If I'm using Berger VLDs (I do for long range hunting), then I compare their published G7 BC with the G7 BC I calculate using my chrono'd MV and 1000 yd velocity. If they're close, that helps improve the confidence in the accuracy of my chronographs. I'll use the Berger BC as well as my calculated BC along with my measured MV, and see if my ballistic program's predicted 1000 yd velocity matches my chrono'd 1000 yd velocity. If it does, I have further verification that my chronographs recorded pretty accurate velocities. If the predicted 1000 yd velocity is off a bit from my chrono'd velocity, I'll tweak the BC until my program predicts my 1000 yd chronographed velocity. After the BC is tweaked (if necessary) so that predicted 1000 yd velocities match my recorded velocities, I then check to see if the predicted 1000 yd drops match my measured 1000 yd drops. If they do, I have further confirmation that my chronographs recorded reasonably accurate velocity. After these steps, I've verified my ballistic program is properly calibrated to accurately predict down-range velocities and drops.

This whole process provides confidence that my ballistic program will now closely predict the proper dope for LR shots under variable environmental conditions and sloped shots. All this in the effort to avoid becoming a victim of the garbage in - garbage out syndrome.

Nothing to it.

My question for you is how do you know you're inputing correct MV into your ballistics program without a chronograph. Do you base your MV on drops? If your muzzle velocity or BC is in error, you can develop a drop chart for one set of environmental conditions and get away with it - maybe. But if you then relocate to a site with vastly different station pressure and temperature, your drop chart will no longer be accurate, and predicted dope from a ballistics program will also be off the mark.

 

[Greywolf18]

Thanks for all the info guys!....guess I need to buy a chrono (or two) next!

 

[MontanaRifleman]

I It would appear that at least one of your two chronographs isn't performing reliably, if I understood your earlier post correctly.

Yup, at least one of them is off to some degree and probably both. One fo my chrony's is a 15 year old ProChrono and the other Ijust got last fall is a Shooting Chrony Alpha. The ProChrono always, without exception so far gives me higer velocities anywhere from about 5 to 60 fps, but usually in about the 20-30 fps range. I thought maybe the Alpha was more accurate, but lately I'm thinking the ProChrono is because of measured drops and its velocities seem to be closer to other data I see. But... the thing is they both kick out weird stuff. I only use them for ball park info.

My question for you is how do you know you're inputing correct MV into your ballistics program without a chronograph. Do you base your MV on drops? If your muzzle velocity or BC is in error, you can develop a drop chart for one set of environmental conditions and get away with it - maybe. But if you then relocate to a site with vastly different station pressure and temperature, your drop chart will no longer be accurate, and predicted dope from a ballistics program will also be off the mark.

It basically comes down to an educated guess which is what I believe that 99% of us use. I start with the published BC and chrono velocity and I will adjust them to match the actual drops.

 

[MontanaRifleman]

Thanks for all the info guys!....guess I need to buy a chrono (or two) next!

If I were you, I would get a CED MII. It's probably the best resonably priced chrony on the market. And if you can find someone with an Oehler to set it up in tandem with to check, that would be a real good idea.

 

[Pdvdh]

Yup, at least one of them is off to some degree and probably both. One fo my chrony's is a 15 year old ProChrono and the other Ijust got last fall is a Shooting Chrony Alpha. The ProChrono always, without exception so far gives me higer velocities anywhere from about 5 to 60 fps, but usually in about the 20-30 fps range. I thought maybe the Alpha was more accurate, but lately I'm thinking the ProChrono is because of measured drops and its velocities seem to be closer to other data I see. But... the thing is they both kick out weird stuff. I only use them for ball park info.




It basically comes down to an educated guess which is what I believe that 99% of us use. I start with the published BC and chrono velocity and I will adjust them to match the actual drops.

If you only hunt in one location at the same elevation, then developing a drop chart at that location is a pretty good method. You'll still have temperature and pressure differences to contend with at that location, but one could adjust for barometric pressure changes and temperatures after the ballistics program has been tweeked to match a drop chart.

I sight in my rifles at Elevation 275', and hunt sheep, goats, black bear, and caribou at elevations up to as much as ~6200' elevation. I also commonly take angled shots. Last fall on a goat at 720 yds on an incline of 30 degrees. The elevation dope was right on the money. First shot was slightly left due to a slight wind.

My method gives me confidence that the input values for BC and MV are very close to the true values. Then I have every reason to place confidence in predicted dope from my ballistics program (LoadBase 3.0) for long range engagements under vastly differing environmental conditions and angled shots.

The chronograph is the best tool I'm aware of for the shooting public to obtain reasonably accurate MV. There are better methods but they're beyond the budget of most.

Three weekends ago a fellow at the range set up a new Shooting Chrony - I don't know what model. I let him shoot over my chrono setup after he had shot and recorded velocities over his ~ 25 times. The velocity obtained over my chronographs was right in the middle of his velocity spread. I also looked at the velocity spreads he was recording over numerous shots. They seemed reasonable. I didn't see any obviously out-of-whack velocities amongst the data.

 

[MontanaRifleman]

Paul, as mentioned in another thread by another member from Montana, we hunt a wide variety of elevations and we can get some angle shots as well. I should probably make myself clear, that I feel that my chrony's are fairly close in determining an average velocity range, but I wouldn't call them precise, and I wouldn't throw out a load because one of them, or both of them, gave me a large ES. The reason being, that they sometimes give out of the ball park readings, and I just cant tell for sure which are inaccurate and which are close. But after shooting several or more strings on different days, i think I can get a reasonable idea of what the actual MV is. My best guess is that it probably lies somewhere between the two chrony's I have and probably closer to the older ProChrono. Just a guess really. And I think that the vast majority of chrony owner shooters out there who are reading this might be surprised if they put their chrony in line with another one and did it on several occasions.

So, I do use the MV I get for starters. If my drops tell me that my BC and/or MV are too high or low, then I will adjust one or both and use those inputs for the appropriate drop and windage output for the conditions. If the BC source is from Brian Litz, I'll put more weight on that and adjust the MV more than the BC. If the BC source is from the manufacturer (except Berger) and it suggesting my inputs are too high, then I'll probably adjust the BC down more than the MV. I would be very hesitant to raise a manufacturer's BC.

In the end, I'm looking for bullets to hit where I expect them to down range. It would be nice to have a program that would graph a shooter's drops at several ranges and then match the drop curve to othere MV and BC curves.