Home Forums Hunting & Shooting Ask The Gunwriters NOVEMBER column--Sectional Density

DOES SECTIONAL DENSITY MATTER?

One of the terms some more technically-minded big game hunters toss around is "sectional density," the relationship of the bullet's frontal diameter to its weight. Sectional density is a major factor in ballistic coefficient, and if you prefer using high-BC bullets matters very much.

It can also somewhat of a factor in terminal ballistics, affecting how deeply the bullet penetrates--though it's not nearly as important as some hunters believe. Or perhaps more precisely, sectional density's effect on penetration varies far more than its effect on bullet flight.

Most hunters use expanding bullets on big game. Typically, the front end of the bullet mushrooms when it enters the animal, flattening and widening until the bullet's shape resembles a typical, actual mushroom, with a stem (the bullet's shank) holding up a broad, rounded cap. However, lead-cored bullets sometimes expand into other shapes, such as irregular balls, or even flatten out, resembling a lumpy flying saucer. In some the stem/shank of the bullet also expands behind the mushroom.

Many monolithic bullets, made entirely of copper or copper alloys, tend to expand into a shape resembling a 4-petaled flower more than a mushroom cap. This is because most monos have hollow-points, rather than "softpoints" where the lead core is exposed. Most have four longitudinal grooves inside the hollow-point to enhance expansion, resulting in four distinct petals.

Sometimes the petals break off during expansion, and while many hunters believe this detracts from "killing power," some monolithic petals are designed to break off, creating pieces of shrapnel that, like the frontal jacket and core of lead-cored bullets, results in more tissue damage.

In fact, we could spend considerable time analyzing and debating the deadliness of expanding bullet designs. But the main point is that ANY expansion changes the sectional density of the bullet., because the frontal area of the bullet increases. Also, usually (though not always) the bullet's weight decreases.

Since both the expanded front and weight-loss can vary considerably, it's obvious that the unexpanded sectional density of the bullet resting in the case neck is a far less important factor in penetration than many hunters believe.

Most hunters also believe bullets retaining a higher percentage of their original weight will always penetrate deeper. This seems logical, but the area of the mushroom has considerable effect on penetration--again because a larger mushroom lowers sectional density, the relationship between a bullet's frontal area and its weight.

This is partly why some bullets retaining a high percentage of their weight don't penetrate nearly as deeply as others--especially some "bonded" bullets, with the lead core firmly attached to the jacket. While this results in higher weight retention, the mushroom tends to be wider than in non-bonded bullets. This wide mushroom resists being pushed through animal tissue, just like your flat palm resists being pushed through water. Make a fist, and your hand will encounter far less resistance.

All of this is easily proven when testing expanding bullets in various kinds of "media," and in fact was mentioned in the 1978 book that probably made more hunters aware of the advantages of "premium" bullets than any other single source, Bob Hagel's GAME LOADS AND PRACTICAL BALLISTICS FOR THE AMERICAN HUNTER.

Back then only two truly premium bullets were reasonably available in the U.S., neither a monolithic, the Nosler Partition and Bitterroot Bonded Core (BBC). Many of today's younger hunters may find this incredible, when so many companies introduce new wonder premiums every year. Even more unbelievable is that only the Partition was loaded in factory ammunition--and not much factory ammo. The 1978 GUN DIGEST only lists Partitions in Weatherby factory ammo--for an additional two bucks a box. This may not seem like much, but considering inflation $2 then is more like $8 now--and in 1978 you could often buy a 20-round box of ammo for $10.

Hagel described his own penetration tests, and reported that while Partitions retained about 65% of their original weight, Bitterroots retained around 90%, a big difference. But he also pointed out something many shooters missed, both then and now: The Bitterroots also expanded much wider, due to their thick copper jackets, while Partitions tended to lose all or most of their soft front core, with what remained of the relatively thin front jacket tended to fold back over the remaining shank of the bullet.

As a result, BBC's did not penetrate as deeply as Partitions of equal weight and diameter. This wasn't necessarily a bad thing, because the wider mushroom of BBC's made a bigger hole inside big game animals, and bigger holes tend to kill quicker, by damaging more vital tissue.

Things have obviously changed since then. Not only have Bitterroot Bonded Cores been discontinued for years, but several of the heavier, larger-caliber Nosler Petitions have the partition moved forward so they'll retain more weight. Those few recovered 9.3mm, .375 and .416 caliber Partitions in my collection (most exit) average 88% weight retention, about as much as BBC's.

The other major change that occurred since then is, of course, petal-type bullets--are not all are monolithics. I started handloading the original Barnes X's in the late 1980's, and while they usually worked well on game, accuracy was often mediocre. Much experimentation eventually resulted in one of the blue-coated XLC's, the 100-grain .25, shooting well enough in a NULA .257 Roberts Ackley Improved for semi-long-range shooting.

However, in the early 1990's Winchester introduced another petal-type bullet, originally called the Black Talon, with a lead core in the rear but a hollow-point, petal-expanding front end. These were more consistently accurate than the early Barnes X's, and eventually became the Combined Technology Fail Sale. Eileen and I used a bunch in both North America and Africa, until the 2004 Barnes break-through of putting circumferential grooves around X-Bullets, solving the frequent accuracy (and copper-fouling) problems. The groove-shanked Triple-Shock X-Bullet (TSX) proved so successful the original Barnes X soon disappeared, along with the Fail Safe.

Of course there are now even more monolithic bullets, and as a group they also demonstrate why sectional density isn't nearly as important in penetration. It didn't take long for many hunters to start using much lighter-weight petal bullets than soft-point lead-cores, because the relatively small "flower" (with gaps between the petals, also reducing frontal area) allowed them to penetrate deeper.

Back in the early days of jacketed soft-point bullets, many hunters considered sectional density of at least .300 to be most reliable on heavier game, bullets such as the 160-grain 6.5mm (.327 SD), 175-grain 7mm (.310), 220-grain .308 (.302), 300-grain .375 (.305), 410-grain .416 (.338) and 500-grain .458 (.341). This is why Elmer Keith (who apparently never did really "get" the Nosler Partition) switched to .33-caliber cartridges instead of the .35 Whelen he used quite a bit in the 1930s: .33 bullets of 275-300 grains had higher section density than .35s of the same weight, resulting not only in deeper penetration but more down-range velocity. However, they still didn't expand as reliably as premium bullets, as shown in the photo on page 30 of his 1946 book, KEITH'S RIFLES FOR LARGE GAME:



Of course, another reason such bullets tended to penetrate deeper was moderate velocity, resulting in less stress on expanding cup-and-core bullets. They were less likely to separate jacket and core, and retain more shank length, which results in straighter penetration. Bullets that open very widely, with little shank remaining, tend to be unstable, not just tumbling but sometimes veering off course.

The petal-type bullets solved such problems with even less weight than Nosler Partitions or various bonded bullets. Nowadays many users of Barnes TSX's (both hollow-point and tipped), Hornady GMX's and Nosler E-Tips drop down in weight for a certain caliber, often choosing bullets with sectional density around .200 to take big game. These not only tend to retain even more of their weight, but the shank of monolithics is longer for their weight, resulting in straighter penetration.

Many hunters who used to pick, say, .30 caliber bullets of at least 180 grains for heavier game often use 130-165 grain bullets--or even lighter bullets of smaller caliber. My wife Eileen's last two elk, for instance, were taken with Barnes Tipped TSX's, one a 100-grain from a .257 Roberts (sectional density .216) and the other a 130 from a .308 Winchester (.196). Despite both being quartering shots--one away and one facing--both dropped promptly to one bullet, with both bullets penetrating several feet of muscle, bone and innards.

Petal-type bullets also tend to exit far more often than lead-cores, especially bonded lead-cores, which normally have not only a wider but very rounded mushroom, which the hide on the far side of an animal tends to "catch" more often. Petal expansion tend to not only be smaller in diameter but the individual petals are sharp-edged. As a result petal bullets exit more often than any other type of expanding bullet, providing that last little bit of penetration valued by many hunters.

So no, high sectional density isn't as desirable as it had been in the past.
This trend started with the Nosler Partition in 1948, and got another boost from the X-Bullet 40 years later, yet many hunters don't understand how those changes affected bullet penetration.





This line-up of expanded premium 7mm bullets is from a test that appeared in the 2006 book, RIFLE BULLETS FOR THE HUNTER: A DEFINITIVE STUDY. They ranged from 150 to 160 grains in weight, and were fired at close range from a 7x57 into dry newspaper, which tends to resemble the results from bullets recovered from big game after hitting heavy bone. Muzzle velocities averaged right around 2700 fps, give or take 80 fps. Please note that retained weight is not directly proportional to penetration, since it's also affected by frontal area.

The bullets are, from left, 156 Norma Oryx (65% weight retention, 8 inches of penetration), 154 Hornady Interbond (72%/8.5), 160 Kodiak (79%/9.0), 150 Swift Scirocco (81%/9.5), 160 North Fork (84%/9.5), 160 Remington Core-Lokt Ultra Bonded (87%/11.5), 160 Swift A-Frame (93%/11.5), 160 Nosler Partition (67%/12.5), and 160 Barnes TSX (100%/13.0).