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 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.

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 CXP2 (deer size) game, while the 150 grain .270 bullet (SD .279) is most appropriate for CXP3 (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 CXP3 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 (CXP1) hunting bullets and their sectional densities:

    .172" (.17) 20 grain, SD .097
    .172" (.17) 25 grain, SD .121
    .204" (.20) 33 grain, SD .113
    .204" (.20) 40 grain, SD .137
    .222" (.22LR) 40 grain, SD .116
    .224" (5.56mm) 55 grain, SD .157
    .224" (5.56mm) 60 grain, SD .171
    .224" (5.56mm) 68 grain, SD .194
    .243" (.24) 58 grain, SD .140
    .243" (.24) 80 grain, SD .194
    .257" (.25) 87 grain, SD .188

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 CXP2 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 (CXP2), such as deer, antelope, sheep and goats:

    .243" (6mm) 95 grain, SD .230
    .243" (6mm) 100 grain, SD .242
    .257" (.25) 100 grain, SD .216
    .257" (.25) 115 grain, SD .249
    .264" (6.5mm) 120 grain, SD .247
    .277" (.270) 130 grain, SD .242
    .284" (7mm) 140 grain, SD .248
    .308" (7.62mm) 150 grain, SD .226
    .312" (.303) 150 grain, SD .220
    .323" (8mm) 170 grain, SD .233
    .338" (.338) 180 grain, SD .225
    .358" (.35) 200 grain, SD .223

As you can see, all of the above have a sectional density over .215 and the average is in the .23's. This is the kind of SD you should look for in a bullet for medium game.


For large (CXP3) game, such as red stag, kudu, elk and moose anywhere in the world, bullets with higher sectional density should be chosen. Good examples of such bullets would be:

    .264" (6.5mm) 140 grain, SD .287
    .277" (.270) 140 grain, SD .261
    .277" (.270) 150 grain, SD .279
    .284" (7mm) 150 grain, SD .266
    .284" (7mm) 154 grain, SD .273
    .284" (7mm) 160 grain, SD .283
    .308" (7.62mm) 180 grain, SD .271
    .312" (.303) 180 grain, SD .266
    .323" (8mm) 200 grain, SD .274
    .338" (.338) 225 grain, SD .281
    .358" (.35) 250 grain, SD .279
    .366" (9.3mm) 250 grain, SD .267
    .366" (9.3mm) 270 grain, SD .288
    .375" (.375) 270 grain, SD .274
    .458" (.45) 400 grain, SD .272

All of the bullets immediately above have a sectional density over .260 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 (.338+), they have proven adequate for hunting thick-skinned dangerous game (CXP4):

    .264" (6.5mm) 160 grain, SD .328
    .284" (7mm) 175 grain, SD .310
    .308" (7.62mm) 220 grain, SD .331
    .312" (.303) 215 grain, SD .316
    .323" (8mm) 220 grain, SD .301
    .338" (.338) 250 grain, SD .313
    .366" (9.3mm) 286 grain, SD .305
    .375" (.375) 300 grain, SD .305
    .416" (.416) 400 grain, SD .330
    .458" (.45) 500 grain, SD .341

Most people will not be surprised to find the heavy .338 to .458 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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Copyright 2001, 2012 by Chuck Hawks. All rights reserved.


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