IS SECTIONAL DENSITY DEAD?
Long, heavy-for-caliber bullets. Elmer Keith based much of his rifle writing on their virtue: 250-grain, 275-grain, even 300-grain .338 bullets. As a successful ivory hunter, Walter Bell made his fortune using long, heavy-for-caliber bullets: 160-grain 6.5mm, 175-grain 7mm, 215-grain .303 (.311), 250-grain ,33s.
These guys, and many others with extensive experience, believed that long, heavy-for-caliber bullets provided deeper and more reliable penetration on game. The comparative term is sectional density (SD), which provides an index that compares a bullet's weight to its diameter. All bullets of the same weight and diameter have the same sectional density.
The calculation is fairly simple. First, convert grains to pounds. There are 7,000 grains to the pound, so divide grains by 7,000. Let's stay with Elmer's 250-grain .338. So 250 divided by 7,000 equals .0357 pound. Next, square the diameter in inches: .338 times .338 equals 0.1142. Now, to derive SD, simply divide the bullet's weight in pounds by the square of its diameter. In this case, .0357 divided by .1142 equals .3126.
Any reloading manual will tell you that a 250-grain .338-inch bullet has an SD of .313, the same number we just got rounded to three decimals. Hopefully, we can agree that a 250-grain .338 bullet is fairly long and heavy for caliber.
An SD around .300 has long been considered a magic number for "high" SD. Other good examples of bullets legendary for penetration include the 160-grain 6.5mm, which has a higher SD of .328, and the 175-grain 7mm bullet, with a slightly lower SD of .310.
The original 215-grain bullet for the .303 British--used a lot by Walter Bell because of ammo availability--has an SD of .317. The 250-grain bullets Bell used in his .318 Westley Richards (diameter .330 inch) had an SD of .327. For comparison, a 200-grain .308-inch bullet has an SD of .301, while the 300-grain .375 bullet has an SD of .305.
There are higher SDs. Keith's beloved 275-grain Speer .338 bullet has an SD of .345. Hornady's 285-grain .338 Match bullets have an SD of .356, and one of Winchester's original loads for the .338 Win. Mag. featured a 300-grain bullet with an SD of .375. The 220-grain .30 caliber (.308) bullet has an SD of .331. lhe 400-grain bullet standard in all the .416s is quite heavy for caliber, with an SD of .330.
Some folks believe tried-and-true bullet weights aren't heavy enough, and although uncommon, "super-heavy" bullets exist in some diameters. A 250-grain .308-inch bullet has an off-the-chart SD of .376. A 350-grain .375 has an SD of .356, and the 450-grain .416 has an SD of .372.
Shooters of my generation were taught to worship at the shrine of sectional density. The question is: What exactly does SD buy you? With bullets of like sectional density, if shape, construction, striking velocity and resistance encountered are the same, then penetration should be about the same. If we kept all these things similar, then a bullet of higher sectional density should out-penetrate a bullet of lower sectional density.
Realistically, it is difficult to keep everything the same--especially that business about resistance. A bullet of smaller diameter naturally meets less resistance than a bullet of larger diameter. If shape, construction, and velocity were similar, I'm totally convinced a 250-grain .338 (SD .313) will penetrate better than a 300-grain .375 (SD .305).
And Walter Bell's beloved 175-grain 7mm (SD .310) probably penetrates better than larger-caliber bullets of similar sectional density--again, provided everything else is the same. But Bell the elephant hunter was primarily concerned with non-expanding bullets that held their shape and penetrated until they exited or came to rest.
We need enough penetration to reach the vitals of whatever size game we're hunting, although some of us prefer exit wounds. Today we have all manner of hunting bullets to choose from with all types of construction: tipped, bonded, homogeneous alloy. Expansion limits penetration because, as diameter increases, there is more resistance.
Some bullets, such as bonded-core and homogeneous alloy bullets, retain much of their weight during penetration. However, a lot of hunting bullets lose a lot of weight during penetration. For deer-size game, I use a lot of plain old cup-and-core bullets that often lose 40 percent of their weight.
Remember the two components of sectional density: diameter and weight. When a bullet expands, its diameter changes, and as a bullet loses part of its jacket or core, its weight changes.
A good non-expanding solid can be recovered with only rifling marks and be so perfect that it could be reloaded and fired again. Its sectional density is what it is. The sectional density of an expanding bullet upon impact, however, will not be the same after a couple inches of penetration--as diameter increases and bullet material is wiped away the sectional density goes down.
I got a note the other day from a veteran shooter who commented that today "an SD of .250 is the new .300." And he's correct. With tough modern designs such as bonded core and homogeneous alloy bullets, you can shave bullet weight and obtain very satisfactory performance. Examples include the 140-grain 7mm bullet, which has an SD of .248--exactly the same as the 165-grain .308-inch bullet. The 130-grain .270 bullet has an SD of .242.
I don't think of these bullets as light for caliber. In these diameters I regularly use these bullets for deer-size game. With tougher modern bullets, I have used these weights for elk-size game with perfect satisfaction.
Sectional density says nothing about bullet shape. Shape has a bit to do with terminal performance in that blunt-nose bullets tend to transfer more energy upon impact and deal a heavier initial blow. They also generally initiate more rapid expansion than sharp-pointed bullets.
Shape is also extremely important to a projectile's downrange performance. Aerodynamics not only affect the trajectory curve but also influence the striking velocity. This translates directly to bullet energy, a formula that uses the square of velocity.
Shape is usually expressed as a ballistic coefficient or BC. It's an index of a bullet's ability to overcome resistance in flight, and like SD, BC is expressed in a three-digit decimal--but the two are not related.
A G1 BC around .500 is still an aerodynamic bullet, but interest in long-range shooting has spawned a whole new array of "low drag" bullets, such as Berger's VLD and Hornady's ELD lines. Some of these are long and heavy-for-caliber with BCs once undreamed of-- not just deep into the ,600s but in some cases into the .700s.
But in any given cartridge, as bullet weight increases, powder capacity and acceptable pressure will reduce velocity. And there are two more important issues. First, standard rifling twists may be too slow to stabilize the extra-long and extra-heavy bullets. Second, you can bump against cartridge overall length restrictions. Long-range shooters solve these issues with faster-twist barrels and use longer actions.
Today's rifleshooters are urged to worship at the shrine of BC, but depending on how they're constructed, bullets with the highest BCs may not be the best choices for hunting. For long-range target shooting BC wins, but not everybody wants to lie prone and get pounded by a .338 Lapua, which, in large part, is why the mild 6.5mms are so popular with long-shooters.
Can you have it all? Of course you can. In most bullet diameters, you can find bullets with reasonably high SDs and high BCs--designed to perform just about any job you have in mind. However, not all of us need to shoot at long range all the time. You want to knock the stuffing's out of a big buck or boar or bear at moderate range? Try a good old round-nose bullet of at least medium weight for caliber. You will be amazed at how visibly hard they hit. A lot of the old "high SD" bullets were round-nosed, perhaps most classic being Speer's 275-grain .338 and Barnes Original's 250-grain .308 in both expanding bullet and solid.
Since I'm more a medium-range hunter than a long-range shooter, I've always paid at least passing attention to sectional density. I have tried extra-heavyweight bullets with ultra-high SDs, including the 250-grain .308, 275gain .338, and, 350-grain .375.
I gave up on them because, in my cartridges, velocities were too slow for my taste and accuracy often suffered, almost certainly due to too-slow rifling twists. However, I would be using them if I thought they were essential. They're not, and I've never had a problem with standard, tried-and-true bullet weights in any of my favorite calibers.
Certainly, I've also paid at least passing attention to BC. But that depends on what I'm doing. For general-purpose work, I'll invariably choose a spitzer bullet of medium to heavy weight for caliber (SD between .250 and .300) with at least reasonable aerodynamics--a Gl-model BC from the lower .400s upward. For instance, for the sable pictured in the lead photograph for this article, I used a 180-grain InterLock in a .30-06--a bullet with an SD of .271 and a BC of .425. It's just a "plain old bullet," but it has worked extremely well on a whole bunch of game.
Long-time favorites include Hornady's old flat-based InterLock in 130-grain .277 and 180-grain .308. The former has an SD of .242 and a BC of .409; the latter has an SD of .271 and a BC of .425. These sectional densities are okay and the ballistic coefficients are pedestrian, but field use out to at least 400 yards is unhampered by the numbers.
I might well choose a more aerodynamic bullet when setting up for big, open country. Recently, I've done most of my open-country hunting with Hornady's 200-grain .308 ELD-X: SD of .315, BC of .626, both high numbers. On the other hand, when I'm not anticipating longer shots, I usually shoot blunt-nose bullets in my .375s--and I often load up with hard-hitting round-nose bullets for hogs and black bears at closer ranges.
Either way, don't get too wrapped up in the numbers. Neither ballistic coefficient nor sectional density speaks to terminal performance in any significant way. However, before we pronounce SD completely dead, allow me to end with a final thought.
Yes, it is a limited concept based on minimal input, and it's really just an index of weight to caliber. Depending on what you're doing, either shape or construction may be much more important. It is thus not a number to obsess on. But as a hunter, there is one thing I am certain of: Bullet weight covers a lot of sins in bullet construction.
Caption: Loaded in radically different cartridges and of different diameters and weights, these four heavy-for-caliber bullets have similar SDs. (l.-r.): 175-grain 7x57 (SD .310), 200-grain .300 Wby. Mag. (.301), 250-grain .338 Win. Mag. (.313) and 300-grain .375 H&H (.305).
Caption: Sectional density is not a constant with expanding bullets. This 300-grain Swift A-Frame .375, recovered from a buffalo, had an SD of .305, but as it lost weight during expansion and penetration, the resulting SD is only .062.
Caption: These three heavy-for-caliber bullets have similar sectional densities, and since they have the same shape and construction, they'll penetrate similarly: (l.-r.) 175-grain 7mm (SD .310), 200-grain ,30-caliber (.301) and 250-grain .338 (.313).
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|Publication:||Petersen's Rifle Shooter|
|Date:||Jul 1, 2019|
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