Home Forums Reloading Big Game Rifles Newbie chronograph questions

Midway had a sale on Chrony F-1's, and I got one. It's their basic model, of course and merely measures velocity. Even though I wasn't a math major, I can figure the average velocity, but I'm not sure about standard deviation. The questions:

1: How do I figure standard deviation?

2: What information from the chronograph is the best indicator of a given load's potential accuracy?

I shot 5 of my most accurate 44-40 load and it went:
1- 1171 (first shot from an oiled bore)
2- 1156
3- 1157
4- 1159
5- 1160

I picked up a Carcano at a gun show this weekend, and when I shot it with some mild reloads, the velocity was all over the place. Anyway, your ideas will be sincerely appreciated. SW

I am impressed by the consistancy of your load. Even with the first shot the standard deviation of your load is 5.39.

The best way to calculate the SD is to use a calculator which has Standard Deviation programmed into it. You don't want to do it by hand, as it envolves calculating the mean, subtracting the mean from each value, and squaring the difference, and adding them together, taking the square root and dividing by the sample size. Do not try do the calculation from the above description, because I don't have the exact formula in front of me, and I just wanted to show you how difficult it is to do by hand.

The only indicator of a given load's accuracy, is found at the target. There are too many variables to predict accuracy from velocity figures.

dodgestdshift was really close without having the formula in front of him. I certainly couldn't have rattled that off without looking it up. Fortunately, my statistics book was readily available, so I grabbed it for a quick look. The procedure is



1. find the average (mean) value of the data set (1160.6 for your five shots)

2. subtract the average (mean) value from each measured value (10.4, -4.6, -3.6, -1.6, -0.6)

3. square that difference for each value (108.16, 21.16, 12.96, 2.56, 0.36)

4. sum the squared values (145.2)

5. divide the sum by the number of values you measured to get the population variance, and by extension the population std deviation (which we'll get when we do Step 6). If you are looking at only a sample of a population (e.g., you were to take 10 bullets out of a box of 100 and want to determine the standard deviation of the sample), then you would divide by the number in the sample minus one (n-1, or for our bullet example 10-1=9) for the sample variance, and by extension sample std deviation. For a situation where you want the standard deviation of your five shots, I think you'll want to divide by the total number of shots (5 for the data you provided). population variance = 145.2/5 = 29.04

6. We calculated either the population variance or the sample variance in Step 5 (depending on whether you used N or n-1). Take the square root of the result of Step 5, and that's the standard deviation. population std dev = 5.39



The much easier way is to use one of the standard deviation functions in a spreadsheet such as Excel. I would guess some of the ballistics software packages available would have similar functionality (i.e., would have a built-in std dev function). The spreadsheet will have both the n-1 and the N standard deviation functions, so be aware of which one you're choosing.



In Excel you would go under the Insert menu and select Function. Click on the Statistical function category on the left side, then select STDEVP for the population standard deviation. You'll get a pop-up box. Across the blank from "Number 1," click on the box, and then select the range of cells for which you want to calculate the std dev. Click OK, and you're finished.



Or you could skip all of the menus and just type

=STDEVP(

into a cell, select the range of cells for which you want to calculate a std dev, and then hit ENTER.



If you have any further questions, feel free to ask because I may have skipped over something or I might could shed a little more light on something. Additionally, some one else may be able to share a little more on the difference between the sample and population standard deviations.



Looks like you're off to a good start with your chronograph!

Ramblin is right on the money. Excel or another spreadsheet is definately the way to go.

Thank you for your detailed replies! I think I understand the process now.

So how importand is standard deviation? For that matter, what is the most valuable information one can get from the chronograph other than velocity?

My original intent was to use velocity readings to determine drop at various ranges with specific bullets using the manufacturer's ballistic tables from their loading manuals. However, if there's data that is useful in determining how to adjust one's reloads, I'm interested in that, too. Thanks! SW

The less deviation in your loads , probably the more accurate they will be. I never go all out figuring the SD, I just look at the spread. It's close enough for what I want, and I just use the average for what my reloads are doing.
However although you have a very low deviation, that particular velocity, might not match the sweet spot on your barrel, so you need to play around with different differnt velocities, seating depths, etc.

When using the F1 Chrony, you need to watch the direct sunlight and may have to put a cover over it. Mine is also not very good with snow on the ground.
I always take my 22 with me to the range,and run afew rounds through the Chrony with it, to make sure it is operating ok

Saddlesore,

Are there particular parameters you look for in the velocity spread? Like, how much is too much? Again, I'm new at this chronograph biz. The 44-40 load above is actually the most accurate I've found in my 1894D Marlin. I tried a lot of Titegroup and Unique loads, but AA 5744 was the best. I don't particularly like that powder due to the granular residue, but accuracy is my goal. Again, thanks for your patience with my basic questions. SW

If I get 25 or less, I'm more than happy. You might try some Hodgon's Universal Clays in that 44-40. It's darn sure clean burning, a tad better than W231, adn even better than the new Uniuq taht they brought out that is suppos eto be clean burning . I use it inall my .357light loads, and my 44 mag range loads. I use 2400 for the .357 for full house oads, and H110 in the 44 mag.

NebrHogger:

Standard Deviation is just one variable in determining whether a load is a good one or not. You cannot predict the final accuracy by just using SD. If you have two loads that give similar accuracy at the target you would probably be better of using the best SD. If you are comparing a small group with a larger SD and a bigger group with a smaller SD, I would probably go with the smaller group, rather than the small SD.

Comparing your velocity increases with the powder charge increases can give you an idea of the efficiency of your load, as you increase powder charge. It can also warn of problems developing if there is a radical change in velocity with a consistant increase in powder charge.

Okay, I think I have a basic idea of how to proceed. I just loaded 50 of 32-20 for my new 1894 CCL using 5 different powders - no loads close to published max and will try to get a little range time in tomorrow before I have to go to work. Thanks for your replies! SW