Sectional density is one of those ammunition terms that sounds super-complicated, but it really isn’t that difficult of a concept to grasp. By definition, sectional density is the ratio of a bullet’s cross-sectional area, relative to its mass.
I think where people go cross-eyed is with the phrase “cross-sectional area.” Basically, if you sliced a bullet across the widest part of its diameter, what you would expose is its cross-sectional area. In other words, its caliber.
See? Not that tough, after all.
The mathematical formula for finding a bullet’s sectional density is: SD = the bullet’s weight in grains divided by 7000 (because there are 7000 grains in a pound) divided by the bullet’s diameter (caliber) squared. For example, calculating the sectional density for a 175gr .308 bullet is 175/7000/(.308^2) = 0.264.
Don’t worry about memorizing that formula – all bullet manufacturers list SD with their bullets’ data sheets.
In layman’s terms, what the sectional density does is put the weight of the bullet relative to its diameter into a numeric form that can be used in other calculations. So, if you have two bullets of the same diameter with one being heavier (and, usually, longer) than the other, then the heavier bullet will have a larger SD. For instance, a 115gr .308 bullet has an SD of .173, versus a175gr .308 with an SD of .264, as calculated previously.
One of the great misstatements that I often hear and read is the phrase, “A bullet with a higher SD will penetrate more deeply than a bullet with a lower SD.” You hear this a lot in hunting circles.
The way it should be phrased is: “If all else is the same, a bullet with a higher SD will penetrate more deeply than a bullet with a lower SD.”
The thing is, when comparing two random bullets of the same diameter, there’s a lot more going on than just differences in weight. How about bullet construction? What about the speed at which they will be fired?
For example, the .38 Special and .357 Magnum both shoot bullets from the same reloading bin – however, the .357 Magnum has the capability of firing those bullets exceptionally faster. Let’s say the .38 Special was shooting a 158gr (.177 SD) expanding hollow point at 750fps, and the .357 Magnum was shooting a 125gr hardball (.14 SD) at 1800fps. I don’t think too many people would be betting on the .38 Special experiencing deeper penetration.
