Sectional density (SD) is a bullet’s mass divided by the square of its diameter:
SD = m / d²
Conventionally it is quoted in pounds per square inch, with mass in pounds and diameter in inches, which produces the familiar dimensionless-looking numbers between roughly 0.1 and 0.35. It is not dimensionless; the unit is just usually left off.
Some examples:
| Bullet | Mass | Diameter | SD |
|---|---|---|---|
| 55 gr .224 | 3.56 g | 5.69 mm | 0.157 |
| 62 gr .224 | 4.02 g | 5.69 mm | 0.177 |
| 77 gr .224 | 4.99 g | 5.69 mm | 0.219 |
| 150 gr .308 | 9.72 g | 7.82 mm | 0.226 |
| 175 gr .308 | 11.34 g | 7.82 mm | 0.264 |
| 220 gr .308 | 14.26 g | 7.82 mm | 0.331 |
Why it matters
Drag acts on the bullet’s frontal area. Inertia acts on its mass. Sectional density is the ratio of the second to the first, so it is a direct measure of how well the bullet’s own momentum resists being scrubbed off by the air.
That makes it half of the ballistic coefficient. Recall BC = SD / i: sectional density supplies the mass-to-area part, and the form factor i supplies the shape part. Two bullets with identical SD but different noses will have different BCs; two bullets with identical shape but different SD will have BCs in the same ratio as their SDs.
The practical consequence is the one every reloader learns: within a caliber, heavier is flatter downrange — not at the muzzle, where the heavy bullet starts slower, but past a few hundred metres, where it has given up less of what it started with. A 175 gr .308 leaves the barrel around 60 m/s slower than a 150 gr, and passes it in retained velocity somewhere around 500–600 m.
Where the limits are
Sectional density is bounded by physics and by the rifle:
- Making a bullet heavier at constant diameter means making it longer, and longer bullets need faster rifling to stabilise. This is the link to twist rate; it is why a 1:12” .223 barrel cannot shoot 77 gr bullets and a 1:7” one can.
- A longer bullet also takes up more case volume at a given overall length, which reduces the powder space and therefore the muzzle velocity available to it.
- Very high SD in a small caliber requires either extreme length or a denser core material than lead.
So SD is not something to maximise blindly. It is one of three things you are trading — sectional density, muzzle velocity and twist-rate compatibility — and the best load for a given rifle and distance is a compromise between them.
A note on terminal effects
Sectional density is often quoted in hunting contexts as a proxy for penetration, and there is real physics behind that: the same mass-per-area ratio that resists air resists tissue. But penetration depends at least as much on construction — whether the bullet expands, fragments, or holds together — as on SD. This site covers external ballistics; treat SD here as an aerodynamic quantity, and take terminal-performance claims from sources that actually test them.