May 5, 2026 · By Diane Johnson
Updated 10 September 2026: a Garmin Xero session comes in as a file rather than over Bluetooth, the platform list is now the one the app ships on, the launch is September 2026, and there is no beta date.
At close range, a few feet per second of muzzle velocity is invisible — the bullet lands where it lands and nobody notices. At distance, that same handful of feet per second becomes inches, then minutes of angle. Velocity consistency is the quiet variable behind a load that prints flat at 1,000 yards versus one that strings vertically no matter how well you hold. Your chronograph measures that consistency, but it hands you two numbers to describe it — standard deviation and extreme spread — and they do not mean the same thing. Reading them correctly is the difference between chasing the right load and chasing a ghost.
Standard deviation measures how far each shot's muzzle velocity typically lands from the string's average. It summarizes how tightly every shot clusters around the average — not just the fastest or slowest, but all of them, weighted together. A low SD means the bulk of your shots are landing close to the mean velocity; a high SD means they are scattered. Because SD takes every shot into account, it is a stable, representative number. Add a few more rounds to a well-behaved string and the SD barely moves. That stability is exactly why it is the better figure for comparing one load against another: it reflects the behavior of the whole population, not an accident at the edges.
Extreme spread is the simplest statistic on the screen: the fastest shot minus the slowest shot. That is its entire definition, and that is also its weakness. ES reflects only two shots in the string — the two extremes — and ignores everything in between. A string of ten rounds where nine are nearly identical and one is a flier will show a small SD and a large ES, and the ES will be telling you about the flier, not the load.
Worse, ES is biased by sample size. The more shots you fire, the more chances you give the string to produce an unusually fast or unusually slow round, so ES tends to grow as the string gets longer even when the load itself has not changed. A three-shot ES and a twenty-shot ES are not comparable numbers. It also makes small samples look deceptively good — fire three rounds, get a tight ES, and you have learned almost nothing about how the load behaves over a real course of fire. SD does not have these problems, which is why it should anchor your decisions and ES should be read as supporting context.
Velocity consistency shows up on paper as vertical dispersion. At long range, drop is steep and unforgiving, so small differences in muzzle velocity translate into large differences in where the bullet lands. A faster round shoots flatter and impacts high; a slower round drops and impacts low. Spread your velocities out and you spread your impacts vertically, producing the vertical stringing that long-range shooters fight constantly. Tighten SD and ES and that vertical component shrinks, leaving you with a group whose height is limited by the rifle and the wind call rather than by inconsistent ignition. The further the target, the more a few feet per second costs you — which is the whole reason precision reloading obsesses over these numbers.
Lowering SD and ES is a matter of removing sources of variation, one at a time. Consistent powder charges come first: precise metering or weighing so that each round burns the same amount of propellant. Uniform primers matter because ignition is where velocity variation is born — a primer that lights the charge the same way every time produces the same pressure curve every time. Brass preparation does the rest: uniform case volume and consistent neck tension mean each round releases the bullet under the same conditions. Taken together, consistent charges, uniform primers, and uniform brass produce consistent ignition, and consistent ignition is what a low SD and a low ES are actually measuring.
LoadOut records each shot from your chronograph — bring a Garmin Xero C1 Pro session across as a .fit file exported from Garmin Connect, or photograph the screen and let the app read it — and computes the numbers that matter for each charge weight: mean velocity, standard deviation, and extreme spread. Because every shot is captured rather than transcribed, you get an honest comparison between loads instead of a hand-picked best string. You can see at a glance which charge holds the tightest SD, where a flier is inflating an ES, and which node is genuinely stable across a real sample. And it stays out of your way: LoadOut is local-first and private, with no tracking — your load data sits on your device, and no LoadOut server ever receives it, because we don't operate one that could. A cloud copy is one you choose to make, encrypted on the device with a passphrase we never see.
LoadOut runs on iPhone and iPad, Android, Mac, and in the browser, with Apple Watch, Wear OS, and Garmin companions, and has a free tier and an optional Pro upgrade. Explore LoadOut →
LoadOut launches in September 2026. Join the waitlist →
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