Sectional Density for Beginners
By Bob Beers
Bullets have several quantifiable characteristics. One of the most important is Sectional Density (SD). It took me a long time to finally grasp its true significance, but once I did, much of my confusion about bullets just disappeared.
Sectional density, according to the SpeerReloading Manual No. 13, is defined as: "A bullet's weight in pounds divided by the square of its diameter in inches." Note that SD is independent of a bullet's shape. All bullets of the same caliber and weight will have the same SD, regardless of their shape or composition.
For the lay person, SD can be considered to be a calculated value that represents how much mass of a given cross-sectional area is necessary to push a bullet through a given medium (such as a game animal).
It's calculated as follows: Sectional Density =
(bullet weight in grains)
As the frontal area (think caliber) of a bullet increases, the weight behind it must increase accordingly to achieve the same penetration.
The value of Sectional Densities for various classes of animal seems to have been empirically derived by the family of hunters over many, many years. Their collective experiences indicate that these (or similar) values seem to work well. Given that a Sectional Density of .180 is good for small animals, .200-.230 is good for medium size animals, .270-.280 is good for large animals, and .300+ is good for larger and tougher animals, then we can make the following table. (I know that some of the small caliber bullets do not exist with the listed sectional densities, but I left the numbers in the table for reference purposes.)
The data in the following table shows that, for a given sectional density, the frontal area and weight of the bullet increase at the same rate. In other words, as the penetration resistance increases (the frontal area increases) the push behind it (weight) increases at the same rate, thus maintaining the expected depth of penetration for a given animal or class of animals.
Note that the sectional density remains constant for a given animal class and as the caliber increases the weight of the bullet must increase to push the larger frontal area of the bullet through the animal. The benefit of the larger calibers is that their bullets make larger holes (wound channels).
As the animals become larger, as we move from small to medium, medium to large, and large to thick-skinned, the bullets become progressively larger as well. From class to class, the sectional density of the bullet progressively increases, which increases the bullet weight, so that the bullet will penetrate deeper into larger animals.
Understanding this concept has cleared up a lot of confusion (for me) about bullets and has provided the means by which I can more easily compare bullets and cartridges. A SD of .180 would be acceptable for hunting varmints (Table 1). A SD of .230 would be good for hunting CXP2 game such as deer (Table 2). A SD of .280 would be appropriate for hunting CXP3 game such as elk (Table 3). And, a bullet with an extreme SD of .330 would be chosen primarily for heavy, thick skinned CXP4 game such as elephant (Table 4).
These tables illustrate the effect of sectional density. I hope that they prove useful as an aid for understanding one of the key properties of bullets.
Copyright 2006, 2012 by Bob Beers and/or chuckhawks.com. All rights reserved.