Should you record weather conditions when zeroing a rifle?

The claim

Somewhere along the way, careful shooters picked up the habit of logging temperature, pressure, humidity, and altitude every time they zero. The logic sounds airtight: conditions affect bullets, the zero is the foundation of everything, therefore the zero's conditions must be preserved. Some ballistic apps encourage it with a full set of "zero atmosphere" input fields.

We put that habit through Wrist Dope's ballistics engine, and the verdict is blunt. At a 100-yard zero, the environmental signal is so far below the noise floor of the zeroing process itself that recording it captures nothing. You cannot measure the effect with your rifle, you cannot dial it with your turret, and no solver can ever make use of it. The notebook entry is ceremony.

This paper is the accounting behind that claim, and the one real exception to it.

How small the signal actually is

Start with the ordinary case, because it is the one that describes your life: the same sea-level range, zeroed on a 90 °F July afternoon, shot again on a 30 °F January morning. Our reference load is a .308 Winchester pushing a 175-grain match bullet at 2,650 fps, with a typical powder sensitivity of 1 fps per °F.

One hundredth of an inch. That is the entire contribution of six months of barometric and temperature change to a 100-yard zero. The other 0.12 inches comes from the powder being cold, which is a property of the ammunition, not the weather report, and we will come back to it.

Now stretch the test to the limits of the planet. Zero on a standard day, then fire that untouched rifle at 100 yards in the two harshest realistic shooting environments on Earth: an arctic sea-level winter at −40° with a strong high (air density 1.55 kg/m³), and a 95 °F afternoon at 10,000 ft of elevation (0.79 kg/m³). The air density between those sites differs by a factor of two, and the whole ballistic solution at 1,000 yards swings 23 MOA between them.

Four hundredths of an inch of air effect, across the full span of conditions Earth can produce. Expressed the way a solver sees it, the air-only difference between those two zeroes is 0.07 MOA. Everything larger in the table is, again, the ammunition.

How large the noise is

Against that signal, here is what the zeroing process itself contains:

• Your turret. A quarter-MOA click moves impact 0.26 inches at 100 yards. The largest air-only shift on Earth is a seventh of one click. There is no setting on your scope that can act on the number you wrote down.
• Your groups.Zeroing means estimating the rifle's true point of impact from a small sample. Standard group statistics put the center of a five-shot group about a tenth of an inch away from the rifle's true mean, on average, for a rifle that shoots honest one-inch groups. Most hunting rifles shoot worse, and the wobble grows with the group size. The measurement error in the zero itself is several times larger than the atmospheric effect you are logging.
• The wind you did not log.A 10 mph crosswind moves this bullet 0.73 inches at 100 yards. A 3 mph breeze you barely noticed contributes more than the entire atmosphere, and nobody's zero notebook has a trustworthy wind entry.
• Everything else. Cold bore versus fouled bore, a warming barrel, aiming error on the target, the target stand flexing. Each of these routinely exceeds four hundredths of an inch.

A recorded value is only data if it rises above the noise of the instrument that measured it and can change a decision downstream. The zero-day atmosphere fails both tests. If you re-zeroed the same rifle twice on the same afternoon, the difference between the two sessions would exceed the difference between zeroing in Alaska and zeroing in the Andes.

The one thing in the notebook that was ever real

Look back at both tables: whenever the shift crept toward a quarter inch, the ammunition's temperature did it, not the air. Powder burns slower when it is cold, muzzle velocity drops, and unlike the air effect this one keeps growing downrange, because a slower bullet falls farther at every distance. Cold ammunition alone moved our load 9 inches at 600 yards.

So the instinct behind the weather log is aimed at something real. It is just pointed at the wrong measurement. The temperature that matters is the temperature of the cartridge, its effect acts through muzzle velocity, and the right way to capture it is a chronograph: measure your load's velocity once near summer temperatures and once near the coldest temperatures you hunt in, and give the solver the resulting fps-per-degree number. That is a property of the ammunition, established in one or two sessions, good for the life of the load. It does not need to be re-logged every time you check zero, and no amount of zero-day weather journaling substitutes for it.

The exception: far zeros

Everything above depends on the zero being at 100 yards, because a bullet 0.12 seconds out of the muzzle has not given the atmosphere time to act. Zero farther out and the air gets a progressively larger vote in your sight setting. Here is what our two extreme environments bake into the zero itself as the zero range grows:

At a 300-yard zero the atmosphere's fingerprint on your sights is ten times what it is at 100, and by 500 it is a genuine error source that a solver can only remove if you logged the conditions. So the honest statement of the rule is conditional. Zero at 100 and the weather log buys you nothing. Zero at distance and you have signed yourself up for the bookkeeping, because now the notebook is the only way to un-bake the weather from your rifle. Given the choice, take the shorter zero and skip the clerical work.

What deserves the ink instead

The zero session produces exactly four durable facts worth keeping:

1. The zero range (make it 100 yards).
2. The ammunition: load or lot, and its chronographed muzzle velocity.
3. The load's temperature sensitivity, from the chronograph, once.
4. The date and round count, for tracking wear and lot changes.

Weather belongs in the ballistic solution at the moment of the shot, where it moves impacts by feet, not in an archive of zeroing days, where it moves them by hundredths of an inch. That division of labor is the entire design premise of a modern solver: you hand it one clean, condition-free measurement of your rifle, it reads the sky in real time from there. Spend your range time making the zero group smaller. The barometer was never the problem.