Sectional density affects every area of a bullet’s performance, but too few people know about it.
Granted, you can be a great shot, proficient hunter, or effective soldier without knowing. But since it’s so important, I think it’s better to know.
Sectional Density simply tells you how concentrated a bullet’s weight is compared to it’s diameter.
Or in layman’s terms:
Sectional Density is how “long and thin” versus “short and fat” a bullet is.
The above bullets are in scale, and almost the same weight (top is 123gr, bottom is 125) But the bottom bullet is slightly larger in diameter, (it’s hard to tell, but true)
It should make perfect sense that the top bullet is more aerodynamic that the bottom bullet.
It should also make sense that the top bullet will penetrate more deeply than the bottom bullet (assuming similar construction)
That’s just common sense, but it’s true because of sectional density.
To calculate a bullet’s Sectional Density, you can either go online and use this nifty Sectional Density Calculator, or you can find the area of the bullet’s cross section, then divide it by the bullet’s weight in grains.
Yeah, I’d rather use the calculator too.
Here are some values for common bullet diameter and weights so you can get a feel for Sectional Density.
|Bullet Diameter||Bullet Weight||Sectional Density|
Note that heavier bullets have a higher Sectional Density.
Something most people don’t know: Sectional Density is one of ONLY two factors that determine how aerodynamic a bullet is.
Only two factors.
The other one is the shape of the bullet.
In most modern spitzer bullets, the bullet’s shape usually accounts for less than 10% of the aerodynamics. That means Over 90% of the bullet’s aerodynamics is determined by its Sectional Density.
It’s that important!
Example: Look at this picture again: (and remember, they weigh almost the same, but the Bottom bullet is wider so they have radically different Sectional Densities)
They are both Hornady SST bullets. The Top one is a 123 grain 6.5mm bullet with a good sectional density of .252. The Bottom one is a 125 grain .308 diameter bullet with an awful Sectional Density of .188.
Here’s what their trajectories look like starting a 2500 Feet Per second: (top line = top bullet, bottom line = bottom bullet)
Here’s what the chart doesn’t show you: At 500 yards the top bullet is moving over 400 FPS FASTER starting at the same velocity! If that wasn’t enough, the top bullet is moved half as much by the wind. (21 vs 41 inches)
Do you think 400 fps would make a difference in bullet drop (and wind resistance) for accurate shooting? Do you think adding 400 fps would make a bullet more lethal? (with good shot placement of course)
And you get all these advantages without increasing recoil!
Are you starting to see how important Sectional density is?
That’s why I love high Sectional Density bullets…
And that’s only HALF the story!
The other half is how the bullet performs once it hits it’s target (assuming the target is something other than paper)
For Hunting, High sectional Densities mean the bullet is more likely to pass completely through the animal, or at least reach the vital zone. High sectional density bullets penetrate more reliably, especially through thick hide and bones.
Did you ever hear the advice to use heavier bullets for heavier/bigger game?
You need those longer and heavier – almost needle like – bullets to get through hair, hide and bone to reach the vital area.
For tactical shooters: Sectional density means greater ability to shoot through cover and greater likelihood of penetrating body armor.
(NOTE: How the bullet is constructed matters more than Sectional Density for performance on targets. Example: the VERY effective light weight, all-copper bullets made by Barnes are famous for deep penetration even with low sectional densities because of how they are made. However, their low sectional densities mean they shed velocity more quickly.)
There’s a HUGE misconception in the shooting world that increasing a Bullet’s Diameter makes it more lethal.
That’s not true.
6.5mm Bullets have been used for over a century to kill Elk, Moose, and other large game using 140 grain bullets (Sectional density: .287) or 160 grain bullets (Sectional density: .328).
For comparison, a .308 bullet would need a 218 grain bullet to match the 160gr 6.5mm bullet’s sectional density, or 190 grains to match the 140gr bullet.
Bullets that heavy produce MASSIVE recoil to reach the velocities a 6.5 can reach with very mild recoil…
And there’s no difference in effectiveness IF you have good shot placement.
African Game might need a very heavy bullet of a slightly larger caliber to punch through the huge animals on the African plains. But 7mm is all you need in North America, an 6.5mm will do almost every thing a 7mm will do with less recoil.
Heavy recoil ruins shot placement. (and you can’t miss fast enough to fill a Hunting tag or win a gun fight)
Low Sectional Densities are why I dislike bullets of .30 caliber and larger. They require bullets that are much heavier to reach the sectional densities of even modest 6mm, 6.5mm or 7mm bullets.
Larger / wider bullets just can’t compete in sectional density without getting heavy.
Heavy bullets need higher recoil to match speed or slower bullets to match recoil. Either way, you’re giving something up to narrower, higher sectional density bullets…. Which are just as lethal with good shot placement.
At close range it’s not very important.
But as the ranges get longer, higher sectional density bullets buck the wind better and retain speed better. Bucking the wind better means more accuracy. And accuracy (plus good bullet selection) is every thing.