Help with Chronograph please


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Jeffery8mm
February 25, 2008, 10:24 PM
Ok guys, I just startd using a Pact chony, bought used, and need assistance.
Please explain SD, ES, and any other pertinant stuff
I know what the abbreviations mean< but what do they mean?

you want the lowest sd right? Any and all help is appreciated.
Thanks
Jeff

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taliv
February 25, 2008, 11:08 PM
SD is a measure of how consistent your velocities are. i.e. on average, how close they are to the mean. i.e. what the curve looks like if you made a histogram

ES is the highest recorded velocity minus the lowest recorded velocity. if you run those through a ballistic calculator, you can see how far apart (in theory) your rounds would be vertically, worst case, at a given range. i.e. if there's 150 fps difference between the top and the bottom, how many minutes difference is there between those velocities at 600 yrds?

you want both of them to be as low as possible.
SD in single digits is excellent. you are unlikely to encounter factory rounds lower than SD of 20-30 fps. (in my experience at least) and some are WILDLY higher than that.

jfh
February 25, 2008, 11:18 PM
Here's a video (http://msdn2.microsoft.com/en-us/library/bb671345.aspx) that may be of help. (I don't know; I haven't viewed it yet).

At it most fundamental level, statistical analysis as applied to reloading is the use of particular mathematical formulas that will indicate a high(er) probability of "repeatability." That usually means something like "If a group of shots demonstrates a low Standard Deviation (from the mean) and a narrow Extreme Spread, then the liklihood is that another group of shots made the same way will perform the same way."

IMO, the key to making Chrono results work include

1) use a minimum of ten-shot strings to get more valid analyses;

2) manipulate one variable at a time;

3) retesting the loads to see the strings are repeatable, and

4) ignoring it all if the load is accurate, is at the performance level you want, and otherwise satisfies your soul.

Jim H.

The Bushmaster
February 26, 2008, 10:12 AM
jfh has it...Read his #4. If the loading is accurate...Who cares what the ES, SD and AD are. As long as the AV (average velosity) is what you desire and the round is accurate. All else is mote...Although I do load for highest AV with the lowest ES.

Jeffery8mm
February 26, 2008, 10:13 AM
Thanks guys!! All this is new to me.

Jfh... AMEN to number 4!!!!!!!!!!!

Jeff

LotI
February 26, 2008, 10:33 AM
Take a look at this months Shooting Times. Excellent article that found that the lowest SD was *NOT* the most accurate out of the particular rifle. Statistics are not the truth sometimes!

Steve C
February 26, 2008, 12:22 PM
Basically SD is how consistent the velocities of your ammo is.

Go to Wikipedia and type in Standard Deviation, it will give you the mathematical definition and explanation. SD is how close your data points (velocity measurements) are to the mean (average velocity). Statistically 68% of your velocities should measure +/- 1 SD from the Mean, 95% will be +/- 2 SD's from the mean.

ES or extreme spread is the difference between the highest and the lowest velocity measurement. Usually the larger the ES the less consistent your ammo is.

For handgun ammo a SD of 9 to 20 fps is common for my reloads ans is pretty accurate ammo. Most all factory ammo I've chrono'd has a SD of around 9 to 12 fps.

Walkalong
February 26, 2008, 02:04 PM
4) ignoring it all if the load is accurate, is at the performance level you want, and otherwise satisfies your soul.

Jim H.
Yep. I have produced many accurate loads over the years before owning a chrono.

I have NEVER chronoed a load from my bench gun, which will put them all in one hole over and over if I don't screw it up.

jfh
February 26, 2008, 03:09 PM
When I got out the chrono last fall to start checking my "replica loads" workups, I found it valuable to determine the actual ballistics of a given recipe in comparison to the base / GDSB 38+P 135-gr. PD rounds.

I've entered the chrono results into a database / excel spreadsheet--but I have not done any close analyses yet. I do know there are several examples of my #4 recommendation. I had figured out the "accuracy correlation. or lack thereof" issue fifteen years ago when I first worked up 'barely Major loads' in .45ACP.

My initial test fires of the GDSB 38+P ammo showed tight SDs and ESs--and good accuracy. Later, when testing again, they were wide--and the ammo was still accurate.

What I am now finding useful is comparing the published data from one powder with another, and gaining more inferred insight about pressure levels. That has enabled me to figure out recipes for powders without published data--for example, WSF for revolver loads. (Speer 14 now has some WSF data, BTW.)

Through its use, I also gained the insight that other tweaks (LOA, crimp) had as much impact as one-tenth grain increments in powder tests.

However, I do see it as just one of a set of tools to use.

Jim H.

Tarvis
February 27, 2008, 04:59 PM
the lowest SD was *NOT* the most accurate out of the particular rifle. Statistics are not the truth sometimes!

+1. Numbers don't lie, the way you percieve them can give you the wrong information however. A chrono will give you velocity information, but accuracy is beyond the chronographs scope.

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