Chronograph ES / SD Statistics
Mean, extreme spread, and standard deviation from a string of chronograph readings.
How this works
These are the same basic statistics any chronograph's software computes, applied here to whatever string of velocities you enter by hand: the average velocity, the spread between your fastest and slowest shots, the standard deviation (a measure of how tightly the shots cluster around the mean), and coefficient of variation (standard deviation as a percentage of the mean, useful for comparing consistency across very different velocity levels).
How many shots make a meaningful string
A 3-shot string can look great by luck and still hide real inconsistency; most serious load development uses at least 5, and often 10 or more, shots per string before drawing conclusions about a load's true consistency. A single unusually low or high shot can swing extreme spread dramatically while barely moving the standard deviation, which is part of why SD is often considered the more trustworthy number.
Frequently asked questions
How many shots should be in my chronograph string?
At least 5, and often 10 or more, for meaningful conclusions about a load's true consistency — a 3-shot string can look great by luck and still hide real inconsistency.
Should I care more about extreme spread or standard deviation?
Many precision reloaders weight standard deviation more heavily, since a single unusually high or low shot can swing extreme spread dramatically while barely moving the standard deviation.
What's a "good" SD for a hunting load?
It varies by cartridge and use case, but many reloaders consider single-digit to low-double-digit SD (in fps) a sign of a well-tuned load — compare your result against other strings of the same load rather than a universal target.
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