There’s an on-going discussion, really an argument, over the relative value of the sectional density of bullets in their penetration of larger game animals. Part-1 of this presentation is a summary of both sides, especially as it relates to big-bore projectiles. But I will also do more than just summarize; we will define the issues more clearly and draw some conclusions based on science and experience in Part II.
The argument has developed because of the introduction of so-called “premium” bullets, and mono-metal bullets, starting way back in the ’40s with John Nosler’s “Partitions” and, since then, the bonded-core and mono-metal bullets.
The position of those who essentially deny that the “sectional density” (SD) of a bullet has any real value in judging it’s penetration potential is based on personal experience with premium, “super-premium” bonded and mono-metal projectiles, as well as the testimony of others with similar experiences. In other words: anecdotal evidence. That’s not all bad evidence… it does suggest something. But the conclusion drawn is false if it says that SD doesn’t matter anymore, OR it has little or no value in predicting penetration, which some believe to be the most important work that a bullet must accomplish. I will show why this position, as defined, is flawed.
On the other side of “the battle” is a dwindling group who tenaciously hold to the “antiquated” idea that sound science plus anecdotal experience doesn’t lie. I stand with this “embattled” group.
Let the games begin!
The current “Conventional Wisdom” side of the war launches “hard-core word-missiles” with this impact: “Who cares about the science of math and what it tells us, we KNOW from experience that TODAY’s bullets, using modern technology, penetrate far better than yesteryear’s bullets of the same or heavier weight (hence higher SD in the same caliber) fired from the same rifles!!!!” (!!!! – to show some heat in the battle…) To some degree they are right, and there is lots of anecdotal evidence to suggest that — like many joining their ranks without ever having fired a shot into a really big game animal themselves! But the problem with it is that they are comparing apples with stones!
So let’s back up for a moment and define words and their relative significance, which might even help the embattled side!
Sectional Density: is the ratio (or relationship) of a bullet’s weight to it’s diameter. In simple terms, the heavier it is in relation to it’s diameter the higher the SD will be. That’s a fact. It’s also a fact (science) that the heavier a bullet is in relation to another bullet of the same caliber-cartridge with equal chamber pressure, the heavier bullet will ALWAYS have more momentum (which most consider to be of greatest importance in the slaying of XL size beasts intent of turning the hunter into compost!) than the lesser-weight bullet.
Example?
Let’s take the ubiquitous .30-06 Springfield to prove our point, and the results would be typical of any cartridge fit for use on big game where a variety of bullet weights might be employed.
The standard .30-06 has a selection of factory loads available, typically ranging from 125grs to 220grs. All factory loads will be restricted to no more than 60,000 psi. Assuming that the following loads are usual factory loads, it should be apparent that the 220gr load will have more momentum than the 125gr load… and as a matter of fact, more momentum than any other load including 180s and 200s.
Momentum: is force, and can be expressed in several ways, but the simplest is by multiplying the weight of the bullet in grains (gr) by muzzle velocity in feet per second (fps). Since that’s a huge number, we’ll divide it by 7000 (the number of grains in a pound). That’s basically Elmer Keith’s version which he called pounds-feet per second, or: lbs-ft/sec. Science or not, that shouldn’t be too difficult to manage. Using the nominal muzzle velocities for the .30-06, we come up with the following results:
125gr = 58 lbs-ft/sec at 3250 fps MV. SD = .188
150gr = 62 lbs-ft/sec at 2910 fps MV. SD = .226
165gr = 66 lbs-ft/sec at 2800 fps MV. SD = .248
180gr = 69 lbs-ft/sec at 2700 fps MV. SD = .271
200gr = 71 lbs-ft/sec at 2500 fps MV. SD = .301
220gr = 76 lbs-ft/sec at 2410 fps MV. SD = .331
Yeah, I know, all these could be improved somewhat from handloading, but the differences would still amount to the same thing. What was the point? That the heavier bullet (with a higher SD)will ALWAYS have more momentum (force) than ANY lighter bullet when fired from the same rifle at equal pressure.
The same principle applies to ANY sporting cartridge used for big game. (Varmint rifles use a variety of bullets but the differences in weight is not usually significant when employed for small game. Construction of bullets has more importance in varmint hunting than a couple more grains of weight.)
Let’s look at results from a true Big Bore… this time my Ruger No.1 in .45-70 IMP employing handloads:
325gr = 121 lbs-ft/sec at 2600 fps MV.
330gr = 123 lbs-ft/sec at 2600 fps MV.
350gr = 125 lbs-ft/sec at 2500 fps MV.
400gr = 140 lbs-ft/sec at 2450 fps MV.
450gr = 149 lbs-ft/sec at 2314 fps MV.
500gr = 157 lbs-ft/sec at 2200 fps MV.
The SD of the respective bullets in .458″
325gr = .221
330gr = .225
350gr = .238
400gr = .272
450gr = .306
500gr = .341
I think it should be obvious by now that the heavier bullet, with the greater SD, has significantly greater momentum, and therefor will penetrate more deeply than the lighter bullet with less momentum and less SD, all else being equal such as bullet shape and construction. That’s based on math (science), as well as experience. It just works that way!
Now, as to why many believe that SD is no longer relevant to penetration, we’ve mentioned it’s due to the unquestioned fact that expanding bullets of the “lead-alloy core inserted into a gilding-metal jacket type” tended to lose 50%, or even more, of it’s total unfired weight upon hitting and during penetration of a big game animal. That was acceptable “back in the day” when such bullets were used on medium game, such as whitetails, as deep penetration was not that much of an issue. And especially was that true when muzzle velocity was relatively modest by today’s standards — around 2200 fps to 2800 fps. And what some refer to as the “cup and core” bullets still manage to cleanly kill many thousands of medium game animals annually, when the right lead-core, gilding metal jacket type bullet is placed in the right spot.
On the other hand, many of today’s hunters buy the expensive monolithic expanding, or premium bonded, or partition-type expanding for hunting elk, whitetails or mulies, swearing they work “best” for those pursuits even though they want “pass-throughs”. In other words, when a bullet passes through a game animal there’s an entrance and an exit — but no retrieved bullet to examine or weigh! The idea being that one can now choose a lighter bullet that will do as well, or better than traditional “cup&core” heavier bullets (in the same caliber), that have higher SDs, therefore SD is now insignificant. Actually, if we only consider the issue from that one-sided stance, any argument is moot — or so they imagine! But, honesty and science requires that we look for truth in the examination of all sides! That approach to the truth will be coming up in Part II.
But, big game, very large game and, especially, dangerous game is another matter. We’ll look at those issues next time as well, and seek to answer the question: Does SD (sectional density) matter when using a big-bore rifle for the hazardous sort or those that can weight a half-ton or more?
‘Til next time…
Shalom
Bob Mitchell