A five-shot chronograph string over a lunch break gives you a number. It rarely gives you the right number, and it never gives you a standard deviation worth quoting. This session is longer, more boring and produces data you can build a season on.
What it measures
- Your rifle's actual average muzzle velocity with a specific lot of ammunition.
- The standard deviation of that velocity, from a sample large enough to mean something.
- The temperature coefficient: how much the velocity moves per degree.
Setup
| Item | Detail |
|---|---|
| Rounds | 20 for session one, 10 for session two |
| Ammunition | One lot. Do not mix lots or you are measuring lot variation |
| Chronograph | Any type; place it consistently relative to the muzzle |
| Thermometer | Needed. A velocity without a temperature is half a measurement |
| Sessions | Two, at temperatures as far apart as you can arrange |
Session one — the baseline
- Record the ammunition temperature before the first shotPut the thermometer next to the rounds, not in the sun and not in your pocket.
- Fire two fouling shots and discard themA clean bore reads differently. These are not data.
- Fire twenty rounds in strings of fivePause between strings so barrel temperature does not climb steadily through the session and add a trend to your data.
- Record every velocity, including the odd onesDo not delete an outlier because it spoils the SD. Note it and look for a cause — a light primer strike, a case you know was different.
- Compute the average and the standard deviationTwenty shots gives an SD you can quote. Write it down with the sample size next to it.
Session two — the temperature slope
Repeat with ten rounds at a materially different temperature — a winter morning against a summer afternoon is ideal. Same rifle, same lot, same chronograph placement.
Divide the difference in average velocity by the difference in temperature. That is your coefficient, in fps per degree, and it is the number that lets a solver adjust your velocity for the day instead of you carrying two DOPE cards.
Scoring it
| Result | Reading |
|---|---|
| SD under 10 fps over 20 shots | Excellent. Velocity is not your limiting factor |
| SD 10–18 fps | Good factory match ammunition. Fine to 800 m |
| SD 18–28 fps | Ordinary. Expect visible vertical past 700 m |
| SD over 28 fps | Look for a cause before blaming the load |
| Coefficient under 0.5 fps/°F | Temperature-stable. One card covers the year |
| Coefficient over 1.5 fps/°F | Sensitive. Enter it in the app and keep it current |
Pass mark
You pass when your rifle profile contains a measured muzzle velocity with its temperature, a standard deviation from at least twenty shots, and a temperature coefficient from two sessions. Not a catalogue figure — yours.
Turning SD into a vertical band
The number that matters on target is not the SD in fps but the vertical spread it produces at distance. Solve your trajectory at your average velocity, then again at average plus one SD, and read the difference in drop at the distance you care about. Double it, and that is the band roughly two thirds of your shots fall inside before the rifle's own dispersion is added.
If that band approaches the height of your target, the load is your limit and better wind calling will not fix it.
Common mistakes
- Quoting a five-shot SD. Sampling luck dominates. Twenty is the minimum for a number you would act on.
- Deleting outliers to improve the number. The tails are where the misses come from.
- Shooting a fast twenty-round string. Barrel heat adds a trend that looks like a velocity property.
- Recording air temperature instead of ammunition temperature. These diverge fastest exactly when it matters — in strong sun or hard cold.
- Mixing lots. A second box may be a different load in every way that counts.
Once you have the velocity, the next step is confirming that the solver's predicted drop matches reality at distance — that is the DOPE ladder.
Frequently asked
Can I do this without a chronograph? +
You can derive a working velocity by truing against real drop at distance, which is covered under truing. It is a good substitute, but it absorbs other errors into the velocity figure, so a measurement is still better.
How different do the two temperatures need to be? +
As far apart as you can manage. Ten degrees gives a slope dominated by measurement noise; thirty or more gives a usable coefficient.
Does barrel temperature affect velocity too? +
Yes, and it is a separate effect from powder temperature. That is why the drill uses strings with pauses rather than one continuous twenty-round burn.