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Discussion Starter · #1 ·
I just got a new Chrony Beta Master chronograph. I loaded two different loads and weighed the powder in each load. After shooting the two different load strings, I had a 34 fps variation in each of the strings. Is that about what I can expect as far as consistancy goes? The only thing that I didn't do was use once fired cases for the test. That may or may not make a difference.
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MD
 

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Keep in mind the industry standard for deviation from design is 90 fps + or -. Means a 1200 fps load could be 1290 or 1110 and still meet SAAMI standards. Obviously most all factory loaded ammo will be far closer than that, but shotguns are not usually as close as match centerfire ammo. I'd say 34 fps is very good indeed. Do about a dozen shots and see how they stack up. Run the spread and the SD. Interesting but meaningless stuff! :roll:

BP
 

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Discussion Starter · #4 ·
The next step is to load the shells using the loader and see what I get w/o measuring each powder charge. At least these first ones gave me a basis with which to compare the next loads.
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MD
 

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Highly simplified, standard deviation is the measure of how spread out and disparate the individual values in a set of data are.

The lower the standard deviation number the closer the values are to each other.

The math formula is complicated but fortunately the Internet is loaded with JavaScript standard deviation calculators.

Here's one of the better ones:

STANDARD DEVIATION CALCULATOR

Your extreme spread of 34 fps would probably produce a fairly low standard deviation.

You should chronograph at least a half-dozen loads to get a decent sample.

The calculator above doesn't list high, low and extreme spread but you can easily do the math on that yourself.

The results produced by any chronograph worth anything should include at least:

Average velocity
High velocity
Low velocity
Extreme spread
Standard deviation

BTW: Chronograph results from measuring both the shot and powder charges to get them exact in each load will give you a good idea of what the load itself is capable of, but that's an ideal situation and the only way to determine real-world values for your loads is to chronograph what the reloading press produces.
 

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I'm not a fan of Shooting Chronys (smashed one once out of sheer frustration), but I'm sure later generations of it produce all the basic data in some fashion or another.

When shooting, keep the muzzle about three feet away from the skyscreens so that muzzle blast doesn't affect the results.
 
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