Bullet Sectional Density Matters
Bullet sectional density matters to hunters because it influences penetration and energy. The higher any bullet's sectional density number, the better it should perform on big animals like elk, moose, bear, kudu, eland, buffalo...
Image of Cape buffalo bull illustrates the kind of bulky animal that requires a high Sectional Density bullet for adequate penetration.
The bigger and tougher the game, the higher the bullet Sectional Density you want in any caliber. The momentum behind a high SD bullet helps it penetrate deep. And that can be important when trying to impress a Cape buffalo.
Now if only we knew what the heck sectional density is! Well, let's find out.
Long and Heavy Equals High Sectional Density
Sectional density is defined as a bullet's weight (mass) divided by its cross sectional area. Do the math and the resulting number is called the bullet's sectional density or SD. In plain English, the heavier a bullet in any caliber (diameter,) the higher its SD. And the higher its SD, the better it will retain momentum. Both in the air and in game.
Stated simply, the longer your bullets in any caliber, the higher their sectional density and the better their chances for deep penetration, all else (bullet materials, construction and velocity) equal.
A line-up of 30-caliber bullets from 100-grain to 165-grain clearly shows the idea behind sectional density.
Fully exposed 30-caliber bullets illustrate the concept of sectional density. It's visually suggestive the longer, heavier 165-grain bullet on far right will penetrate farther than the short, round-nosed plinker on the left (given the same velocity.) There is simply more mass in the long bullet's shank to continue driving it forward regardless how much the nose does or does not expand.
How to Double SD
From another perspective, consider a 100-grain .308 bullet (they exist!) and a 200-grain .308 bullet. Obviously both are .308" in diameter. And just as obviously one is twice as heavy as the other. Since the diameter didn't increase, the length must have. It did. Given the same base and nose contours, the 200-grain slug should be twice as long as the 100-grain.
You mathematical geniuses have probably already figured out the SD numbers should also have doubled. They did. Hornady's 100-grain .308 bullet scores SD .151. Its 200-grain ELD-X is rated SD .301. (I don't know why the balance is off by 1. Probably one of those mathematical rounding-off deals.)
Sectional Density Consistent Despite Bullet Shape
In any weight and caliber, SD remains consistent regardless bullet shape. Sierra's 200-grain Spitzer Boat Tail and its Hollow Point Boat Tail both rate SD .301. So does Nosler's 200-grain Partition. Swift's 200-grain A-Frame, Norma's Oryx, Barnes' X, Speer's SP Hot Core... All 200-grain .308 bullets have an SD of .301. But that doesn't mean they all retain the same downrange energy, as we'll see next.
Swift Scirocco and A-Frame bullets in 180-grain, .308 illustrate how Sectional Density remains the same regardless the different shapes. But the sleeker Scirocco will retain more energy downrange due to aerodynamic efficiency.
These
have different shapes, but because both weigh 180 grains and both are .308", they have the same Sectional Density. The sleeker, more aerodynamic shape of the Scirocco means it will retain more energy at all ranges and fly flatter, but not necessarily penetrate deeper because of variable jacket thickness, lead hardness, etc.
Don't Confuse SD with B.C.
Do not confuse these SD numbers with B.C. numbers. Ballistic Coefficient numbers differ based on not just mass, diameter, and length, but shape. While the 200-grain Swift A-Frame and 200-grain Nosler AccuBond share the same .301 SD, the A-Frame's B.C. number comes in at .444 while the AccuBond's is rated much higher at .588. This is because the Accubond has a boat tail base, a sharper tip, and a more steeply tapering nose curve or ogive.
Sleek bullet shape, i.e. high B.C., helps projectiles of any SD retain energy because they slip through the atmosphere rather than fight it. So high B.C. aids and abets SD. The higher both numbers, the more energy is delivered at all ranges and the more a given type of bullet should penetrate. (More details on B.C. in this blog.)
Equal weight, equal caliber Swift bullets of different shapes and materials have identical SD.
While both these Swift A-Frames (left) and Sciroccos have the same weight, caliber, and sectional density numbers, they will not necessarily penetrate equally due differences in hardness, jacket thickness, etc. The more aerodynamically efficient Sciroccos will retain more energy at all ranges, but they will likely expand more widely, increasing drag and reducing penetration. They might also lose more mass due to lead erosion while the A-Frame protects more shank lead behind an internal wall of copper jacket material.
This is why choosing a round-nose or flat-point over a spire point is questionable logic. I used to think, as most round-nose fans probably do, that these fire-plug slugs hit harder. They just look as if they would, right? But obviously they don't because they burn so much of their energy pushing air before ever reaching the target. I suppose this matters little or not at all out to 200 yards or so, but still... Wouldn't you want to maximize retained energy?
If you have a thing for high on-target energy, you might want to pay attention to B.C. as well as SD.
A round nose and spire point bullet of equal weight and caliber have equal Sectional Density.
Even though these bullets are the same caliber and weight, they will not both deliver the same energy downrange or penetrate equally well (or poorly.) The round nose slug's poorer aerodynamic shape (measured as B.C.) wastes energy pushing air while the sharply tipped, tapered spire point minimizes air drag and retains more energy to put on target. Still, variable hardness and nose expansion could influence penetration, too. Neither of these bullets is sized or shaped for deep penetration.
Practical Application of Sectional Density for Hunting
At the risk of redundancy, let's wrap up this treatise by reiterating: the higher a bullet's sectional density and Ballistic Coefficient rating, the more energy it should retain at all ranges. The higher its SD, the more mass it will keep in the shank behind the mushroomed nose. And the more mass behind the mushroom, the more momentum will be maintained to push that tissue-tearing nose deep and, ideally, right out the far side, leaving a sizable hole from which blood can flow to lay an easily followed trail.
But don't assume you need high-end SD bullets for all hunting. The reason .308 bullets -- and most other calibers -- come in a variety of weights is to give you options. Lighter bullets fly flatter and faster. They generate less recoil, too. And for smaller game like pronghorns, whitetails, and sheep, may be all you need.
High sectional density is not required for all game. A thin skinned, lightly boned, 120-pound impala ram can be dispatched with a fairly light, low SD bullet. This one from
in Namibia fell to a single 60-grain Federal Fusion from a Savage M110 in 223 Remington. Bushnell
. Bushnell
Ron Spomer tests guns, ammo, optics and assorted hunting gear and writes about them on a variety of websites, blogs, and print media such as Sporting Classics, American Hunter, Outdoor News, and Outdoor Life.
