Our approach
Load Lab exists to make factory ammunition comparable. Every figure on the site is either a number the manufacturer published or one derived from published numbers by a formula shown below. Where a manufacturer doesn’t publish something, we say so rather than fill the gap with a guess.
This page documents how that works. Load Lab is a comparison and education tool — it is not reloading data, and nothing here should be used to assemble ammunition. See What Load Lab is not.
Where the numbers come from
The load database is built from factory-load specifications published by the ammunition manufacturers — muzzle velocity, energy, bullet weight, test-barrel length, and so on, as listed on their product pages and catalogs. Each load is entered and reviewed by hand.
A “factory load” is a complete cartridge sold ready to shoot. That is distinct from a custom handload, which you can build yourself in the Custom Load Builder using your own components and chronograph data.
Caliber and bullet diameter
A cartridge doesn’t use a single bullet diameter — it uses a small family of them that shooters treat as interchangeable. A .45 ACP, for instance, is loaded with both .451″ and .452″ bullets. So when you pick a projectile for a load, the list shows every diameter in that caliber’s family, and each row is labeled with its exact diameter so you can tell them apart.
Those diameters are grouped from a hand-curated table, not by rounding to a tolerance. Real bullet diameters sit too close together for arithmetic to separate them safely: .308″ and .311″ are only three thousandths of an inch apart, but they belong to different cartridges — .308 Winchester and .303 British — and their bullets are not interchangeable. A numeric “close enough” rule would mix them, so the table only groups diameters that are genuinely the same caliber.
When velocity isn’t published
Not every manufacturer publishes a muzzle velocity for every load. When one doesn’t, Load Lab shows “Not published” instead of inventing a figure. Those loads stay in the database — you can browse them, search them, and see their bullet and box details — but they are held out of anything that needs a velocity to be meaningful: side-by-side comparison, the ballistics solver, Find My Load, and sorting by velocity or energy.
We don’t estimate the missing number. Muzzle velocity is a product of the maker’s specific powder charge and test barrel, not something you can infer from the cartridge and bullet weight — the same cartridge and bullet can vary by more than 150 fps between manufacturers, which is over a minute of angle at 500 yards. A guessed velocity that someone then dopes a rifle from would be worse than no number at all.
Energy and sectional density are calculated
Muzzle energy and sectional density aren’t stored — they are calculated from their inputs every time they are shown, so they can never drift out of step with the velocity, weight, and diameter they come from.
- Muzzle energy (ft-lbs) = bullet weight (grains) × velocity (fps)² ÷ 450,436
- Sectional density = bullet weight (grains) ÷ (7,000 × bullet diameter (inches)²)
The energy constant, 450,436, is the value the ammunition industry uses — it folds in the grains-to-pounds and gravity conversions — and we verified it against thousands of loads that publish both velocity and energy.
A calculated energy can sit a fraction of a percent off a manufacturer’s own published figure, because makers round the velocity they publish and some use a slightly different energy constant. The gap is far too small to affect a comparison. Sectional density, which depends only on bullet weight and diameter, matches published values to the printed digit.
The ballistics solver
Trajectories are computed by numerically integrating the projectile’s flight (a fourth-order Runge–Kutta method), using the G1 or G7 drag model — standard drag curves published by JBM Ballistics — according to the bullet’s ballistic coefficient. Atmosphere follows the ICAO standard model.
Unless you change them, the solver uses these reference conditions:
- Sea level, 59°F, 29.92 inHg, 0% humidity
- 100-yard zero, 1.5″ sight height
- 10 mph full-value (90°) crosswind
It calculates drop, retained velocity and energy, time of flight, and wind drift (using the standard lag-time approximation). It does not model spin drift or the Coriolis effect — corrections that matter only at long range and that would demand far more precise inputs than a comparison tool has. Treat the output as a high-quality comparison and planning aid, and confirm your own dope on paper.
What Load Lab is not
Load Lab is not load data. It does not tell you what powder, primer, or charge to use, and its factory-load figures are not a substitute for a current, published reloading manual. Always verify any figure against the manufacturer before you rely on it, and follow established safety practice when handling ammunition and firearms. The tool is provided for comparison and education.