Does ammo temperature affect velocity?

The mechanism

Why cold ammunition shoots low

Smokeless powder is a chemical reaction, and reactions run slower when they are cold. A cartridge that has been sitting in a truck bed at 10 °F builds pressure more slowly than the same cartridge off a summer bench, the peak pressure is lower, and the bullet leaves the muzzle with less speed. Nothing about the bullet, the barrel, or gravity has changed. Only the launch velocity has, and a slower bullet spends longer in the air, which gives gravity longer to work on it.

The trade goes both ways. Ammunition left in the sun on a 100 °F range day leaves faster than it did on the day you chronographed it, and your holds are then a little too much rather than a little too little.

Be precise about what the variable is: it is the temperature of the powder inside the case, not the number on the weather app. A cartridge is a slow soaker. Ammo carried against your body stays near body temperature on a freezing morning, a box left in the shade lags the afternoon warm-up by an hour, and a round that sits chambered in a hot barrel picks up heat the forecast knows nothing about. Air temperature only matters once the ammunition has had time to become that temperature.

The number

Velocity sensitivity, in fps per degree

The effect is quoted as a rate, called velocity temperature sensitivity, because it is close to linear over the temperatures people actually shoot in: so many feet per second of muzzle velocity gained or lost for each degree Fahrenheit the ammunition warms or cools. A load rated at 1 fps per °F that chronographs 2,650 fps at 59 °F will chronograph about 2,591 fps at 0 °F.

We ran four of them through the engine, 0.2 through 1.5 fps per °F. The low end is what a powder engineered against temperature is reaching for, and the high end is what a powder that was not can look like. As rates go they sound almost identical. Watch what they do to the muzzle across a temperature span any hunter sees in one season.

The flat load gives up 12 fps between a 59 °F bench and a 0 °F morning. The sensitive one gives up 88, which is more velocity spread than most factory ammunition shows shot to shot.

The cost

What that does to your impacts at 600 yards

Muzzle velocity in fps is abstract. Inches on a target are not. For this chart the rifle is zeroed at 100 yards on a 59 °F day, the turrets are never touched again, and the shooter dials the 600-yard come-up they wrote down that afternoon. The only thing that changes is the temperature of the ammunition in the chamber. Everything below the center line is a bullet landing under the aim point.

A deer's vital zone is roughly 8 to 10 inches tall. On a 0 °F morning the 1.5 fps/°F load lands 9.4 inches low, which puts a perfectly executed heart shot into the brisket. The 0.2 fps/°F load lands 1.2 inches low, which nothing alive would notice.

The cost, by distance

It is nothing at 300 and a yardstick at 1,000

Velocity error compounds with time of flight, so the same 59-degree swing costs you almost nothing up close and a great deal far out. This is the whole picture for one cold morning: a rifle zeroed at 59 °F, fired at 0 °F, on dope taken at the zero temperature.

At 300 yards even the worst load is 1.9 inches low, inside the group size of plenty of hunting rifles. At 1,000 it is 37.9 inches, longer than a yardstick, and no amount of good wind reading recovers it.

The good news

Your 100-yard zero survives the winter

At 100 yards the bullet is in the air for about a tenth of a second, which is not long enough for a velocity difference to turn into a visible drop. We ran the worst case in the study, a 1.5 fps/°F load carried from its 59 °F zero to a −20 °F morning, and here is the entire damage.

That is the shape of the whole problem. Temperature does almost nothing to a short zero and a great deal to a long shot, so the fix is never to chase your turrets around the seasons. It is to keep the muzzle velocity your solver is working from honest.

Powder

Which powders hold their velocity

Powder makers who engineered for this say so in their own product copy, and that is the only source worth quoting. Hodgdon sells a line it calls Extreme and describes Varget as an extruded propellant that "is insensitive to hot and cold temperatures." It says it modernized H4350, also an Extreme powder, by shortening the grains "and making it insensitive to hot and cold temperatures." IMR lists "insensitivity to temperature changes" among the features of its Enduron powders, and Vihtavuori markets the N500 series on temperature stability.

Read those claims for what they are: a manufacturer saying it worked on the problem. Insensitive is not immune, and none of them publishes a figure in fps per degree for your barrel length and your charge weight. A powder whose data sheet says nothing about temperature is not making a promise in either direction, so treat it as unmeasured rather than bad.

And if you shoot factory ammunition, as most people do, the box will not tell you what is inside it at all. That leaves one honest way to find your number.

Measure it

Two chronograph sessions and one division

You do not need a temperature chamber, a lab, or even two seasons. Because the variable is cartridge temperature, you can make both temperatures happen in one afternoon. Put ten rounds in a sealed bag in a cooler on ice for an hour. Warm ten more to about 110 °F, a sealed bag on the dashboard in the sun or in a tub of warm water does it. Check each batch with an infrared thermometer on the case just before you load, then shoot each string over the chronograph immediately, within a minute or two of leaving the soak, before the cases drift back toward air temperature. Divide the velocity difference by the cartridge temperature difference and you have your load's number the same day you thought of it.

One safety line: warm gently and stop near 110 to 120 °F. Ammunition makers warn against storage much past 150 °F, so no ovens, no heat guns pointed at cartridges, nothing you cannot hold your hand on. The spread between an iced case and a sun-warmed one is 60 degrees or more, which is all the math needs. If you would rather not stage any of it, the two-season version works exactly the same way: a warm-day string, a cold-morning string, and one division.

Two rules make the answer trustworthy. Record the CASE temperature from the infrared thermometer, not the air, because the case is what the powder feels; a cartridge carried in a coat pocket reads body temperature no matter what the forecast says. And use the same chronograph in the same light for both strings, so you are measuring the powder rather than the instrument.

What to do

Keep the velocity honest, leave the zero alone

1. 1

   Chronograph near the temperatures you shoot in.

   A velocity measured on a July bench is a July number. If you hunt in November, get a November string. One extra range trip in the cold buys you every number on this page.

2. 2

   Update the muzzle velocity on your load when the season turns.

   Wrist Dope keeps muzzle velocity on the load, editable on the iPhone or straight from the watch with the Digital Crown. Change that one value to what you measured in the cold and every hold recomputes from it, on the spot.

3. 3

   Leave the turrets where they are.

   The worst case in this study moves a 100-yard zero a quarter inch. Re-zeroing for the season throws away a good zero to chase something smaller than your group.

4. 4

   Treat a big swing as a reason to confirm at distance.

   If you moved 50 degrees and 300 yards from where your data came from, a confirmation shot on a known-distance target is worth more than any correction you can calculate.