Warm propellant burns faster, builds pressure faster and pushes the bullet out quicker. Cold propellant does the opposite. The result is a muzzle velocity that varies with the temperature of the cartridge — not the air, the cartridge — and a trajectory that shifts with it.
How big is the effect?
It depends entirely on the propellant. Modern temperature-stable powders can hold velocity to a few feet per second across a wide range. Older or more sensitive propellants can move on the order of one to two feet per second per degree Fahrenheit, which turns a 50 °F swing into 50–100 fps.
| Sensitivity | Velocity change over a 40 °C / 70 °F swing | Practical meaning |
|---|---|---|
| Low (temp-stable powder) | Tens of fps | One DOPE card covers the year to mid range |
| Moderate | 50–80 fps | Visible vertical shift past 500 m |
| High | 100 fps or more | Separate summer and winter data required |
Factory ammunition rarely publishes a sensitivity figure, and identical-looking boxes from different lots can behave differently. Measuring is the only way to know your own number.
Measuring your own coefficient
- Chronograph a ten-shot string at a cold ambient temperatureLet the ammunition sit at that temperature for at least half an hour first. The cartridge must be cold, not just the air.
- Record the average velocity and the temperature togetherOne without the other is not a data point.
- Repeat on a hot day, or after warming the ammunition deliberatelyThe wider the temperature gap, the more reliable the slope you extract from it.
- Divide the velocity difference by the temperature differenceThat is your sensitivity, in fps per degree. It is a property of that load and that lot.
- Enter it in the app and set the current powder temperatureThe solver then adjusts muzzle velocity for the day instead of you keeping a mental table.
Cartridge temperature is not air temperature
This is where the practical errors live. Ammunition takes time to reach the temperature around it, and it happily reaches temperatures the air never sees.
- A magazine on a black bonnet in the sun can run far above ambient. The first shot after that is not the load you tested.
- Rounds in a trouser pocket in winter are warmed by you, not by the weather, and can be twenty degrees above the air.
- A round chambered in a hot barrel heats fast. In rapid strings this is a real source of vertical.
- Ammunition just out of a cold vehicle has not equalised with a warm range yet, and will not for a while.
The habit that fixes almost all of it: keep ammunition in the shade, out of contact with hot or cold surfaces, and give it time to settle before you shoot data you intend to keep.
How it interacts with air density
A hot day does two things at once. Air density drops, which reduces drag, and powder velocity rises, which flattens the trajectory further. They push the same way, which is why a summer zero can be visibly high in winter.
These are separate inputs in the solver for a reason: air density affects the whole flight, and velocity affects the launch. If you fold them together into one fudge factor you will be right at one distance and wrong at every other. See density altitude for the other half of the picture.
When it is safe to ignore
Inside 300 m, with temperature-stable ammunition, the shift is smaller than your group and you can forget it. Past 600 m, or with an unknown propellant, or on a hunt where the first round is the only round, it deserves the ten minutes it takes to measure.
Frequently asked
Does temperature affect the bullet or the powder? +
The powder. The bullet's flight is affected by air density, which is a separate effect covered under density altitude. Both change with temperature, which is why the two are easy to confuse.
Do I need to re-zero between seasons? +
At 100 m, usually not — the shift there is small. What changes materially is the drop at distance, so re-confirm your far data rather than your zero.
Is a temperature-stable powder worth paying for? +
If you shoot the same load across seasons and care about first-round hits at distance, yes. If you shoot inside 300 m, it is solving a problem you do not have.