Sectional Density

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 stays the same for all bullets of the same weight in the same caliber; shape does not affect SD.

SD is important because it has a significant effect on penetration. Other things being equal (like impact velocity, bullet design and expansion, 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.

Probably the best way to compare different calibers is by SD, not bullet weight. Comparing calibers by bullet weight can be deceiving. For example, the .270 Winchester and .30-06, which are based on the same case, can both shoot 150 grain bullets. However, the 150 grain .30-06 bullet (SD .226) is best used for Class 2 (deer size) game, while the 150 grain .270 bullet (SD .279) is most appropriate for Class 3 (elk size) game. The 150 grain .270 bullet should actually be compared to the 180 grain .30-06 bullet (SD .271), as both of these bullets are appropriate for Class 3 game in their respective calibers and boast similar SD's. This is important to remember when comparing rifle bullets.

Here are some typical small game, varmint and small predator (Class 1) hunting bullets and their sectional densities:

Looking at those figures illustrates why the military has had so much problem with the penetration of 5.56mm NATO (.223 Rem.) bullets in combat. Human beings are Class 2 size animals and none of these .224" bullets have adequate SD for the job, let alone for penetrating barrier materials and light armor. It also shows why none of these varmint weight bullets, regardless of the velocity at which they can be driven, should ever be used to shoot deer and other medium size game animals.

Here are some typical hunting bullets and their sectional densities that are recognized as effective for medium size big game animals (Class 2), such as deer, antelope, sheep and goats:

As you can see, all of the above have a sectional density of at least .201 and the average is in the .23's. All of the small bore calibers (.32 and smaller) have SDs of at least .205. This is the kind of SD you should look for in a bullet for medium game.

For large (Class 3) game, such as red stag, kudu, eland, elk and moose anywhere in the world, bullets with higher sectional density should be chosen. Good examples of such bullets would be:

All of the bullets immediately above have a sectional density of at least .250 and most exceed .270. Bullets of this sectional density, if adequately constructed, have proven able to penetrate deep into large game animals.

I did a quick survey of the hunting bullets with SD's over .300 available in common factory loads and to the reloader in the various rifle calibers. These are the top calibers and bullet weights for maximum penetration. In medium and big bore calibers (.33+), they have proven adequate for hunting thick-skinned dangerous game (Class 4):

Most people will not be surprised to find the heavy .33 to .50 caliber bullets on the above list. However, many may be surprised to find that heavy weight bullets for the small bore 6.5mm, 7mm, .30, .303 and 8mm calibers are right in there with the best medium and big bore bullets in terms of SD. This may help explain why these small bore calibers are so versatile.

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