Understanding Sectional Density
Sectional density (SD) is a standardized value that compares a projectile's mass to its cross-sectional area. It is a foundational metric in ballistics because it describes how a projectile concentrates its mass behind a fixed frontal footprint, which in turn governs how it retains momentum against a resisting medium.
The Formula
Sectional density equals the projectile's mass in pounds divided by the square of its diameter in inches. Grain weights are converted to pounds by dividing by 7,000, since there are 7,000 grains in one pound. The result is a dimensionless-looking ratio, conventionally reported to three decimal places, that lets analysts compare projectiles of very different weights and calibers on one common scale.
Why Shape Is Ignored
SD deliberately excludes nose profile, boat-tail, and meplat geometry. It treats the projectile as though its mass were spread evenly behind a circle equal to the bore diameter. This makes SD a pure mass-to-diameter relationship rather than an aerodynamic measure, which is precisely why it is a building block for the ballistic coefficient rather than a substitute for it.
Relationship to Penetration
A higher sectional density concentrates more mass behind each unit of frontal area, so for a given impact velocity and construction, it generally correlates with deeper penetration. The projectile faces less deceleration per unit of mass as it pushes material aside, which is why long, heavy-for-caliber designs are favored where depth matters.
Caliber Independence
Because diameter appears squared in the denominator, narrowing the bore while keeping mass constant raises SD sharply. This is why a heavy small-bore projectile can outperform a lighter large-bore one in penetration potential, even when the larger projectile weighs more in absolute terms.
Worked Example
Consider a 150-grain projectile of 0.308-inch diameter. First convert mass: 150 divided by 7,000 equals 0.0214 pounds. Then square the diameter: 0.308 times 0.308 equals 0.0949 square inches. Dividing 0.0214 by 0.0949 yields a sectional density of about 0.226. A 180-grain projectile of the same diameter would compute to roughly 0.271, showing how added weight at constant caliber lifts the value.
A Common Misconception
Many readers assume a higher sectional density automatically means a flatter trajectory or less wind deflection. It does not. SD says nothing about aerodynamic shape, so two projectiles with identical SD can fly very differently downrange. Flight efficiency is captured by the ballistic coefficient, which combines SD with a form factor describing the projectile's actual profile.