Sectional density (SD) is a bullet’s weight divided by its cross-sectional area. It’s a single number that tells you how well a bullet can push through resistance, whether that’s air in flight or tissue on impact. A higher SD means the bullet concentrates more mass behind a smaller front, giving it greater potential to penetrate deeply and resist wind drag over distance.
How Sectional Density Is Calculated
The formula is straightforward: divide the bullet’s weight by the square of its diameter. In the shooting world, weight is measured in grains and diameter in inches, producing a unitless decimal number that typically falls between .100 and .350 for common rifle bullets.
For example, a 180-grain .308-caliber bullet has an SD of .271. A 140-grain 6.5mm (.264″) bullet comes in at .287. That 6.5mm bullet, despite being 40 grains lighter, has a higher sectional density because its mass is packed behind a narrower cross-section.
The key insight: only weight and diameter matter. Bullet shape has no effect on sectional density. A 200-grain .308 spitzer boat tail and a 200-grain .308 hollow point both score .301, because they share the same weight and the same diameter. Shape affects aerodynamics, not SD.
What SD Tells You About Penetration
A bullet with high sectional density concentrates more momentum into a smaller area, which drives it deeper into a target. Think of the difference between pushing your fingertip into clay versus your whole palm. Same force, but the fingertip goes deeper because the area is smaller. SD captures that relationship mathematically.
Within the same caliber, heavier bullets always have higher SD. A 100-grain .308 bullet scores .151, while a 200-grain .308 scores .301, exactly double. Since the diameter stays the same, all that extra weight translates directly into more mass per unit of frontal area.
This is why, given equal velocity, a longer and heavier bullet in the same caliber will generally out-penetrate a shorter, lighter one. The heavier bullet carries more momentum behind the same-sized hole it’s punching through material.
SD and Ballistic Coefficient
Sectional density is one of two ingredients in a bullet’s ballistic coefficient (BC), the number that predicts how well it cuts through air. The formula is simple: BC equals sectional density divided by a form factor that accounts for the bullet’s shape.
A bullet with high SD starts with an advantage in flight because there’s more mass pushing forward relative to the air resistance acting on its front. But shape matters too. A sleek, pointed boat-tail bullet with moderate SD can outperform a blunt round-nose bullet with higher SD, because the streamlined shape creates less drag. Two Swift bullets in .308, both 180 grains with identical SD, will perform differently at long range if one has a more aerodynamic profile. SD gets you partway to understanding flight performance, but ballistic coefficient tells the full story.
Common SD Values by Caliber
Here’s how SD scales across popular rifle calibers:
- .22 caliber (.224″): 45-grain at .128, 55-grain at .157
- 6.5mm (.264″): 120-grain at .247, 140-grain at .287, 160-grain at .328
- .30 caliber (.308″): 150-grain at .226, 165-grain at .248, 180-grain at .271, 220-grain at .331
- .375 caliber (.375″): 270-grain at .274, 300-grain at .305
Notice that 6.5mm bullets punch above their weight in SD. A 140-grain 6.5mm (.287) beats a 180-grain .30-caliber (.271) despite being 40 grains lighter. This is a big reason the 6.5 Creedmoor and similar 6.5mm cartridges have become so popular: their bullets achieve excellent sectional density without requiring heavy, high-recoil loads.
SD Guidelines for Hunting
For deer and antelope, many experienced hunters recommend an SD of at least .200. For larger, tougher animals like elk and moose, .250 or higher is a common benchmark. These aren’t hard rules, but they reflect the reality that bigger animals require deeper penetration to reach vital organs.
Most popular hunting loads already fall within these ranges. A standard 150-grain .308 Winchester load has an SD of .226, comfortably above the deer threshold. A 140-grain 6.5 Creedmoor at .287 handles elk-class game. The .375 H&H with a 300-grain bullet at .305 is a classic choice for African dangerous game, where deep, straight-line penetration through heavy bone and muscle is essential.
Why SD Changes on Impact
Here’s the catch that experienced shooters understand: the SD number stamped on the box describes the bullet before it hits anything. The moment a bullet strikes tissue and begins to expand, its effective sectional density drops. A mushrooming bullet doubles or triples its frontal diameter, dramatically increasing the cross-sectional area while the weight stays roughly the same. That means less mass per unit of area, which slows penetration.
If a bullet fragments, SD drops even further because pieces of mass are peeling away. This is why comparing SD between bullets of different construction types can be misleading. A bonded-core bullet that expands to 1.5 times its original diameter and retains 95% of its weight will have a very different effective SD during penetration than a varmint bullet that fragments into a dozen pieces.
The practical takeaway: SD is most useful when comparing bullets of similar construction. Two controlled-expansion bullets with similar starting SD will generally behave similarly in tissue, regardless of caliber. But comparing a frangible varmint bullet to a solid copper mono isn’t meaningful, even if both start with the same SD number.
Copper Bullets and Sectional Density
Monolithic copper bullets add another wrinkle. Copper is less dense than lead, so a copper bullet of the same weight as a lead-core bullet needs to be longer to make up the mass. That extra length can create stability problems in standard barrel twist rates, which is why many copper bullet offerings are lighter than their lead-core equivalents in the same caliber.
A lighter bullet means lower SD on paper. A 130-grain copper .308 has an SD of about .196, well below the .271 of a traditional 180-grain lead-core .308. But copper bullets typically retain close to 100% of their weight after impact, while lead-core bullets may shed 30% to 40%. That near-perfect weight retention partially compensates for the lower starting SD, and in practice, copper bullets often penetrate more than their SD number alone would suggest. Hunters in the UK, for instance, have found that a 130-grain copper .308 with an SD of .196 performs reliably on deer when the bullet expands properly and holds together.
This highlights that SD is one piece of the terminal performance puzzle. Bullet construction, impact velocity, and expansion behavior all interact with sectional density to determine how a bullet actually performs in the real world. SD gives you a useful starting point for comparison, especially among similar bullet types, but it’s not the whole story.