SD and ES in Load Development: What’s a Good Number?

Everyone chases a single-digit SD. The honest answer: standard deviation matters more than extreme spread, and it only matters as much as your distance demands. Here’s how to read both numbers without fooling yourself.

Your chronograph hands you a velocity for every shot. Extreme spread (ES) and standard deviation (SD) are just two ways of summarizing how much those velocities scattered. They’re the most-quoted numbers in load development — and the most misread. A good SD on the wrong number of shots tells you nothing; a great SD at 100 yards barely shows on paper. Let’s sort out what each one is actually worth.

ES vs SD — what each one tells you

Extreme spread is the simplest: your fastest shot minus your slowest. It’s easy to picture, and it’s the number one bad round completely wrecks. Shoot a tidy string and clip one flier, and your ES doubles while the load itself didn’t change.

Standard deviation describes the whole string, not just its two worst members. It’s the average distance of your velocities from their mean — so it tells you what the load tends to do, and one flier nudges it instead of detonating it. Chase SD. Treat ES as a flier-detector: if ES is wide while SD is tight, you probably had one bad round, not a bad load.

So what’s a “good” SD?

As a rough community yardstick for a centerfire rifle load: a single-digit SD is excellent, under ~12–15 fps is solid for most shooting, and once you’re north of ~20 fps you’ll see it on target at distance. But those numbers are meaningless without two pieces of context: how many shots you measured, and how far you’re shooting.

The trap nobody mentions: sample size

An SD calculated from 3 or 5 shots is mostly noise. Standard deviation needs a real sample before it settles down — figure 10 shots as a minimum, and 20–30 to trust it. A “single-digit SD” off a 3-shot string is usually luck, and chasing it will send you re-loading to beat a number that was never real. If you’re comparing charges in a ladder with a few shots each, read SD as a soft signal, not a verdict — and confirm your finalists with a longer string.

Why SD really matters: vertical at distance

Here’s the part that makes SD worth the trouble. Velocity variation turns into vertical dispersion downrange — a slower bullet drops more by the time it gets there, a faster one less, and that spread opens up the longer the flight. At 100 yards, even a sloppy SD hides inside the group. At 600 and beyond, SD is your vertical. That’s why the F-Class and long-range crowd obsess over it and the 100-yard benchrest crowd cares less: SD is really a prediction of how tall your group gets when it counts.

Where it fits in your load decision

SD is one input, not the whole answer. A load can post a pretty SD and still string vertically on paper, or shoot tight while its velocity wanders — so the number that matters is how all of it lines up: the velocity flat-spot, the SD, the mean radius, and the vertical on target. You can square these up by hand in a spreadsheet, or let the math do it.

That’s exactly what Loadscope does: it reads your chronograph CSV, computes velocity SD (and ES) for every charge you tested, lines it up against your group data, and weighs SD into the best-load call instead of leaving you to eyeball it. See how the scoring works →