Sectional Density And Why You Should Bother Knowing About It
One of the most important aspects about bullets in terms of performance is sectional density, and it happens to be one of the attributes hardly anyone pays attention to. Sectional density is the greatest predictor of penetration ability, which is a key component of ammunition performance.
After all, if it doesn’t get far inside the target…it’s no good as hunting or self-defense ammunition…usually, anyway.
How does sectional density work, though? Let’s get into that.
Sectional Density Is A Ratio Of Mass To Cross-Sectional Area
Sectional density is the ratio of cross-sectional area to the mass of an object. A simpler way to put that is how much mass is distributed along a particular axis. This matters, as it establishes how well that object can deal with resistance.
This matters, as the higher the sectional density, the better a bullet is able to penetrate things such as barriers, game animals and hostile personnel.
The simplest formula for it is SD = M/A, or mass divided by a predefined cross-section of an area of an object. However, it’s a little different for bullets.
For bullets, sectional density is calculated like so:
SD = Mass (in grains) / 7000 X bullet diameter in inches²
So, to plug actual numbers into that…
Let’s figure out the sectional density of a 150-grain projectile in .308 caliber, which is the .308/.30-06/.300 Win Mag family.
Bullet diameter is .308, which squared is 0.094864.
Multiply 0.094684 by 7,000 = 664.048
So, 150 gr/664.048 = 0.226. Should you be curious, 55-gr .223 has an SD of 0.158.
Why Does Sectional Density Matter?
Why sectional density matters is that the mass of an object relative to its size relates to its ability to overcome resistance, especially when it comes to barriers such as tissue. The higher the SD, the better it will be able to enter the target…usually.
This is (partially) why flying ashtrays like .45 ACP in the 1911 and .45 Colt in single-action revolvers were prized as self-defense rounds for so long and why many shooters attest .45 ACP and other big rounds to be a superior defense round to 9mm to this day. The bigger bullet is better able to penetrate soft tissue, the claim goes, and therefore makes a better implement of self defense.
This is also why larger calibers were, for the longest time, said to be better for big game hunting than smaller ones. The gunwriters Elmer Keith (favored sectional density and velocity over…everything; helped invent .338 Winchester Magnum, .357 Magnum, .41 Magnum and .44 Magnum) and Jack O’Connor (downed everything from coyotes to black bear with .270 Winchester, championed good bullets and placement) bitterly hated each other because of this particular debate. Well, and some other things.
What has been found, of course, is that while there is something to be said for that, good placement and a good bullet design more than makes up for mere sectional density alone.
Handgun Sectional Density Is Not What You Might Expect
So, again, is sectional density everything its cracked up to be? Not necessarily. Handgun sectional density paints a bit different picture than one might expect.
For instance, you’d think 9mm sectional density is much smaller than that of .45 ACP. It’s actually a narrower gap than you might think.
A 115-gr 9mm has an SD of 0.130. By contrast, a 230-gr .45 has SD of 0.162. Yes, a good amount of difference…but 147-gr 9mm has a SD of 0.167.
What this means is that sectional density increases the heavier a bullet weight is for a given caliber. Oh, and in case you should wonder, 10mm is still left behind by heavy magnum revolver loads. Again, it’s a very impressive round in an autoloader, but the magnum revolvers remain top dogs.
What it also means is that if you believe in sectional density as a desirable trait, which not everyone does, a smaller chambering with a heavy for caliber bullet will actually do just as well as a larger bullet loaded light- to medium-for-caliber.
What’s a good sectional density for self-defense?
The general rule is that humans are a Class 2 animal. We’re about in the same class as deer, mountain lions, wolves and so on. It’s recommended that Class 2 game be hunted with ammunition having a sectional density of 0.2 or above.
The heaviest 10mm is just under that, and the heaviest loadings of .45 Colt, .454 Casull and .44 Magnum meet or exceed it.
Now, we know that even .22 LR can be effectively used on humans with good placement though isn’t 100 percent effective. Then again, even 12-gauge 00 buckshot has failed to get a one-shot stop in real world encounters.
Therefore, it can be said that while high sectional density can certainly predict what will hit hard and deep, it definitely isn’t everything. Bullet selection and placement – meaning accuracy and precision – still matter a great deal. Arguably more than anything else.
About The Author
Born in southeastern Washington State, Sam Hoober graduated in 2011 from Eastern Washington University. He resides in the great Inland Northwest, with his wife and child. His varied interests and hobbies include camping, fishing, hunting, and spending time at the gun range as often as possible.