Understanding Bullet Sectional Density
With the new calibers available for deer hunting many Hoosiers are wondering what type of bullets they should be using this fall.
Without getting too deep into the weeds of selecting hunting bullets, lets explore one of the more important factors in bullets selection; section density.
I learned why bullet sectional density is important during a recent bear hunt. I was in Ontario sitting in a tree stand when a nice black bear came in. At twelve yards distance I placed the red illuminated cross hairs on the bear's front shoulder and squeezed the trigger. The impact rolled the bear over into heavy brush where it laid kicking and bawling like a mad bull. Amazingly, it got up snarling and snapping at its shoulder, then fell over again. It repeated falling and standing as it rolled through the brush until suddenly it gathered its wits and ran off into the deep woods. Hours of searching by the outfitter and other hunters failed to find a speck of blood or the bear.
So what the heck happened? My weapon that trip was a 20-gauge slug gun. In the months leading up to the hunt I had practiced until I could produce three shot groups of less than one inch at fifty yards. I had chosen aiming for the shoulder on the recommendation of other bear hunters with the idea that the slug would break the shoulder, enter the rib cage, and destroy the heart and lungs.
I'm not one to just let things go. I have relived that moment over a hundred thousand times. I want to know why things happen or don't happen.
In my research since then I have discovered sectional density and how it applies to hunting bullets.
The job of a bullet is to penetrate thick hair, skin, bones, and destroy important organs. To achieve that goal a bullet needs good sectional density.
Sectional density (SD) is the ratio between a bullet's weight and its cross-sectional area. Heavier bullets in a given caliber have a higher SD than lighter bullets. For example, a .308 caliber 150-grain bullet is going to have an SD of 0.226 while a .308 caliber 180-grain bullet will have an SD of .271. All things being eqaul, the higher the SD, the father the bullet will penetrate.
Longer bullets of the same caliber and material will have a higher SD.
The fact that heavier bullets penetrate deeper into game is not new science. However, just being a heavy bullet doesn't mean it is going to have good penetration. The slug’s weight was more than double that of a 150-grain bullet fired from a 30-06, and the muzzle energy was equal. It’s all related to its SD.
Look at it this way. My big-game meat cleaver measures six inches by three inches, is 0.125 inches thick, and weighs 7.4 ounces. If I turn the cleaver sideways and try to cut a steak with the flat side, I can swing the cleaver until I'm leaning against the counter gasping for breath and I still will not have penetrated the steak. But, if I use the cleaver as intended all I need is one chop to cut the steak through. Why? The answer is obvious but it also shows how SD relates to penetration. The cleaver's weight, velocity, and energy didn't change. But using the cleaver side-ways I had an extremely low SD of 0.022 and using the cleaver correctly I had an extremely high SD of 0.520!
So what is a good SD? That varies for what you're hunting for. Some experts recommend an SD over 0.200 for thin-skinned game like deer and antelope and an SD of 0.250 for bigger game like elk and moose.
Using the same weight of bullets at the same velocity, a 7mm bullet with have greater SD than a .308 bullet because it has a smaller cross-sectional area.
Going back to my shotgun slug, by taking its weight in pounds and dividing it by its area in inches, I get an SD of 0.183. That's far below what experts recommend for anything other than varmints!
Bears have big bones and inches of fat before even getting to vital organs. Judging from the SD of the slug, I think the soft lead hit the bone, perhaps breaking it, while being flattened in the process. The mushrooming of the slug would lower the SD even more. With inches of fat covering the rib cage behind the leg, the slug probably didn't make it any farther.
So, knowing about SD I decided to spend more of my hard-earned wages on another bear hunt. This time I brought my 30-06 loaded with 180-grain bullets that have an SD of 0.271, right in the recommended range.
This time the shot presented was slightly quartering away so I placed the cross hairs just behind the shoulder and squeezed the trigger. The higher SD bullet did what I expected. It penetrated the rib cage, destroyed both lungs, exited the rib cage, entered the far leg, and exiting it again to plow dirt in the forest of Ontario. I got my bear.
Now, you’re not going to find a bullet’s SD rating on the ammo box, and it isn’t the only characteristic to use when choosing the right bullet. (You can find important bullet data in any good reloading manual.) The shape and structure are equally as important. But by studying why sectional density is important, we can get equal or better performance from even smaller calibers, but I’ll leave that for another time.