Change the ballistic coefficient by five per cent and a 600 m solution moves a little. Change the muzzle velocity by five per cent and it moves a lot. Velocity is the dominant input, it varies from rifle to rifle with the same ammunition, and unlike BC it is directly measurable with equipment that now costs less than a case of match ammunition.
The box number is not your number
Factory velocities are measured in the manufacturer's test barrel, which is usually longer than yours and always different from yours. A rough working figure is 15–30 fps per inch of barrel for typical rifle cartridges, but the spread around that is wide enough that the correction is a guess, not a calculation.
Chamber dimensions, throat erosion, bore condition and the specific lot of powder all move the number further. Two rifles of the same model, same barrel length, firing the same box, can differ by 40–60 fps. That is why the catalogue figure belongs in the app as a placeholder, not as truth.
What standard deviation measures
Standard deviation describes how tightly a string of shots clusters around its own average velocity. For a roughly normal distribution, about two thirds of shots fall within one SD of the mean and about 95 per cent within two.
| SD (fps) | Reasonable verdict | Practical consequence |
|---|---|---|
| Under 8 | Excellent, usually handloads | Velocity is not your limiting factor |
| 8–15 | Good match ammunition | Fine to 800 m for most purposes |
| 15–25 | Typical decent factory ammunition | Vertical spread becomes visible past 700 m |
| Over 25 | Hunting ammunition or a problem | Expect noticeable vertical at distance |
The important caveat: an SD calculated from five shots is not a reliable estimate of anything. Sample size dominates. A five-shot string can easily return an SD of 6 for a load that genuinely runs at 15, and people then buy a whole case on the strength of it. Ten shots is a minimum for a number worth quoting; twenty is meaningfully better.
Why extreme spread is a weak statistic
Extreme spread is the fastest minus the slowest shot. It uses two data points and throws away everything in between, and it can only ever grow as you shoot more — a twenty-shot ES will exceed a five-shot ES from the same load, every time, purely because you sampled more of the tails.
That makes ES useless for comparing strings of different lengths, which is exactly what people use it for. Quote SD with its sample size, and treat a single wild ES as a prompt to look for a cause — a bad primer seat, a case that was not trimmed — rather than a property of the load.
Turning velocity spread into vertical dispersion
This is the number that actually matters on target. A faster shot arrives sooner, so gravity has less time to act on it, so it lands higher. The effect is small close in and grows with time of flight.
The clean way to see it: run your solution twice, once at your average velocity plus one SD and once at minus one SD, and look at the difference in drop. That spread is the vertical band roughly two thirds of your shots will fall inside, before your rifle's own mechanical dispersion is added.
- Enter your measured average velocityTen shots minimum, at a temperature close to what you will shoot in.
- Solve for your target distanceNote the drop.
- Add one SD to the velocity and solve againNote how much less it drops.
- Double that differenceThat is your ±1 SD vertical band at that distance. Compare it with the height of your target.
For most factory ammunition the band is a few centimetres at 500 m and grows quickly past 800. If it is comparable to your target height, no amount of correct wind calling will save you — the load is the limit.
Velocity is not fixed across the year
Powder burn rate changes with propellant temperature, and the same ammunition can lose or gain a meaningful amount of velocity between a July afternoon and a January morning. Temperature-stable powders reduce it; nothing eliminates it. This is covered in powder temperature sensitivity.
The practical habit: record the temperature next to every velocity measurement, and keep the ammunition out of direct sun and off a hot bonnet before you shoot it. A chronograph number without a temperature is half a measurement.
Frequently asked
How many shots do I need for a meaningful SD? +
Ten as a floor, twenty for a number you would base a purchase on. Five-shot SDs are dominated by sampling luck and routinely mislead in both directions.
My chronograph disagrees with the box by 80 fps. Which is right? +
Yours, for your rifle. The box describes a test barrel. Enter your measured figure and confirm it against real drop at distance.
Does a low SD guarantee small groups? +
No. Low SD removes one source of vertical dispersion. Bullet quality, bedding, barrel condition and the shooter remain, and at short range they dominate entirely.