Analyzing chronograph data?

[2000Z-71]

I finally broke down and bought a chronograph. I ended up with a CED M2 unit that I took out to the range today for the first time with my 7mm mag and 270. I have the data downloaded from shooting my strings, velocity, high, low, spread, standard deviation, etc. Just looking at it on the surface, there doesn't appear to be any direct relationships between consistencay (ie: a low spread or standard deviation) and group size.

So what should I be looking for in the data? What is considered to be an acceptable spread or standard deviation? Or am I just trying to over analyze this and should be happy with a small group and the knowledge of its velocity?

Thanks in advance.

 

[Erich]

Or am I just trying to overanalyze this and should be happy with a small group and the knowledge of its velocity?

There you go!

You're exactly right - sometimes wonderfully accurate loads have big spreads. It's all just info to help you get where you're going - you decide where that is. If a bunch of us decided that we wanted our bullets to be fast, accurate and shiny in flight, we could rig up a device to measure the albedo of the bullet as it flew to the target after it left the barrel. Doesn't mean that the info would be of any use or importance to the next guy. You might not at all care about the info from a chronograph, and you might not want one. Or, you might want to shoot the fastest/most accurate/most consistent load ever . . . in which case, you'll need the chronograph to help you in load-development.

Personally, I figure that I want to make sure that 1) my loads are traveling with enough velocity to impart proper damage on the other end, but 2) I want them to hit accurately from my guns. Measuring velocity helps me on the first part, and the deviation/spread info gives me something to consider when things aren't going right with either the first or the second part. I'll take another reading with the next loading (when I alter the first to try to get something better suited for my needs), and then think about what can be interpolated from the new data.

 

[dla]

I'm one of those guys who rarely shoots longer than 100yds. So ES doesn't matter much to me. Add another zero to that distance and I would care a lot. My most accurate 45-70 load had a wide ES, yet it was a oneholer at hunting distances.

With a Chronograph I can tell the effect of different primers, different crimps, etc. I can tell if my loads are in the ballpark of published data. I like that feeling of control - it adds to my sense of satisfaction when reloading.

 

[Pisgah]

Theoretically, a low extreme spread and even lower standard deviation should give the best groups. In fact, it often seems that almost the opposite is true -- and I'm danged if I can even begin to explain why.

If the velocity is high enough to do the job, and the accuracy is there, I have ceased worrying over the numbers much.

 

[bobsdad]

"Theoretically, a low extreme spread and even lower standard deviation should give the best groups. In fact, it often seems that almost the opposite is true -- and I'm danged if I can even begin to explain why."

This sure is a stumper isn't it? But....when I used to be involved with industrial process improvement and experimentation, we found that outcomes are usually affected by many inter-related factors. I guess you could say that the Chronograph is indicating there are other important factors affecting group size. That consistency of velocity isn't an overwhelming factor?

 

[Alk8944]

The most useful information from a chronograph is average velocity. Extreme spread and standard deviation are interesting statistics, but, as mentioned by others, in large are generally of no practical value.

Average velocity allows you to make useful exterior ballistic computations such as remaining velocity and energy, and bullet drop at defined points down-range, if you have the software to do this. Load From a Disc is a good one.

Another interesting thing is to compare relative velocities with the same load in different guns. I frequently will take several revolvers of the same caliber to chronograph a batch of ammunition. It is fascinating to see how often a revolver with a wider flash gap is faster than one with a tighter gap. Likewise, sometimes a longer barrel is slower than a shorter one. I have seen the same load give a Standard Deviation of a single digit in one gun, and as much as in the 70s with another, same day and all conditions equal, shot just minutes apart. Go figure. And these aren't 1-5 round samples, generally at least 10 and, sometimes, much larger just to get a more significant sample.

Using a powder that fills a rifle case 90% or more and being sure to not exceed the average velocity of equivalent factory ammunition determined by shooting it in the same gun also will assure you that you are not exceeding the pressure limits of the cartridge by a significant or dangerous amount. You also can know you are getting standard performance from your rifle for hunting. Isn't a lot of point shooting 7x57 velocities from a 7mm Magnum, or .30-30 velocities from a .300 Magnum just because you don't really know what your ammunition is doing.

Don't over-think it. Velocity is useful. SD & ES seem to indicate consistency, but it doesn't necessarily indicate superior performance, regardless of the parameter by which you are judging.

 

[armadillo]

I have used the older CED millennium for about 4 years. I have noticed the same phenomenon.

For most rifle loads I weigh each powder charge on an electronic scale. My .223 Ackley gives sub 1/2" groups with 100+fps ES. My 30-06 and other large capacity cartridges get 1" groups with ES in the teens.

 

[flat top]

The ES/SD readings on a chronograph exhibit ballistic uniformity of a load at a given distance from the muzzle.....normally at 10 to 15 feet or so is where most of us set up. What happens after that (group size) is more of a depiction of bullet flight characteristics (barrel twist with varying bullet weights, bullet length, shape, etc), sighting error, and environmental conditions which all play a factor in accuracy. I have had some dead nuts benchrest competition loads that didnt read worth a hoot on a chrono, and I have also had some very sloppy groups that gave great ES/SD readings! I use my chrono "after" I find the most accurate load for a given firearm. Like others said above, if the velocity is where it needs to be with that accurate load to get the job done, that is all I worry about. Also, I use the chrono to document velocity increases as I work up a load. Its nice to know that in a given firearm load, that 4 tenths of a grain will give me an "X" amount of velocity gain, and by checking increasing powder charges against your chrono'd velocities you can keep an eye on "listed" pressures from your reloading manuals...not that they would be exactly the same for your individual firearm...call it a "ballpark pressure check". Info like that really helps with continuing load development. I wouldnt worry too much about it. If your gun is accurate and the velocity is where it needs to be, you are good to go!!!!

 

[OKFC05]

I teach math and statistics, but I'm not going to try to do a statistics lesson here. There are a few points worth keeping in mind:

1. You need a minimum of ten shots for the SD to mean much of anything.
2. Your test is a statistical sample, and small differences in SD from batch to batch mean nothing ( are within statistical limits of significance).
An SD of 9 vs 18 is not twice as good, maybe not even better. Now 100 vs 9, I would look for a reason that was happening.
3. If you want to be sure how to interpret SD, take a statistics course and learn how to do significance tests. A flipped coin is 50/50 to land heads or tails, but if you expect to flip a coin 10 times and get 5 heads and five tails every time, you don't know anything about probability and statistics.

 

[R Cubed]

I work my rifle reloads from a light charge to a stronger charge until I find that sweet spot where my groups got tighter and tighter then started getting bigger. I tuned loads for that specific rifle. Chrono data is good to know for flight trajectory. Then the SD and MV are factored in with the software I use.

As already mentioned the bigger the sample, the better - I use a twenty round sample, allowing the barrel to cool between rounds. Sometimes I look back at my data thinking I should have captured the temp, wind, barometric pressure, and altitude also.

 

[Erich]

Rick, I note the altitude, barometric pressure and temperature - I don't see how wind speed affects anything a chronograph would measure . . . and I don't shoot for groups in any kind of wind.