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Here's Chuck Hawk's take on SD:
The Sectional Density of Rifle Bullets

By Chuck Hawks



Sectional density (SD) is the numerical result of a calculation that compares a bullet's weight to its diameter. To calculate a bullet's sectional density divide the bullet's weight (in pounds) by its diameter (in inches), squared. The higher the SD number the better the SD, and the heavier a bullet is in proportion to its diameter.

SD is important because it has a significant effect on penetration. Other things being equal (like impact velocity, bullet design and material, etc.) the higher the SD number, the better the bullet's penetration. In other words, a skinny bullet of a given weight tends to penetrate better than a fat bullet of the same weight, because it concentrates the same force on a smaller area of the target. For example, if other factors are equal, a 150 grain .270 bullet will penetrate better than a 150 grain .35 caliber bullet.

Penetration is important because the bullet must get well inside an animal to disrupt the functioning of its vital organs. A bullet that fails to penetrate the fur, skin, muscle, and bone necessary to reach the vital organs is very unlikely to bring an animal down.

SD stays the same for all bullets of the same weight in the same caliber--shape does not affect SD. This information is important to remember when comparing rifle bullets.


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