Sectional Density vs Terminal Sectional Density

mcb said:

If sectional density was useful then if I give you the following data you should be able to tell me which pernitrates better.

Bullet 1: SD = .192
Bullet 2: SD = .242

But you can't without me providing significantly more data. Even if I gave you the weight and diameter (the minimum data needed to calculate the SD) of Bullet 1 and 2 you still would not be able to tell me which would pernitrate better.

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If we can assume similar design and construction, we can safely assume Bullet 2 is going to penetrate better.

First and foremost, there is no simple answer and none of it is black & white. There is no single number that tells us the whole story. Unlike kinetic energy numbers, at least SD is useful, for something.

SD is a useful number but it must be kept in proper context. It tells us nothing all by itself. SD is useful in comparing similarly constructed/designed bullets of differing diameter, to each other. Where I use this most is with hardcast lead revolver bullets but it also applies to rifles. For example, I know that an LBT style bullet in the SD range of .25-.27 will tend to penetrate the deepest. That holds true whether it's a .44 bullet or a .500. Less than that and you give up penetration. Any more and you're in the realm of diminishing returns. That means 355-360gr for .44's and .45's, or 430's for .475's. The .500's tend to get by with less. Lighter bullets can equal penetration if they have a smaller meplat, such as comparing LFN's to WFN/WLN's. Here we're assuming that similar bullets will have similar rates of deformation if the impact velocity is comparable. In other words, proper context.

All that goes changes when copper/bronze solids come into the picture. Here a lighter bullet exceeds the penetration possible with a solid lead bullet, because there is no deformation. I had an argument with a misguided soul who thought copper solids rendered SD moot. Wrong. To say that SD doesn't matter is to say that weight/momentum don't matter and we know this is false. Otherwise, we could use heat treated aluminum bullets and get the same result but we know that isn't going to happen. Copper/bronze solids don't change the importance of weight to penetration, it just moves the scale back.

TKO is the same way, context is everything. You don't use it to compare expanding bullets to solids, or high velocity bottleneck cartridges to big bores. You only use it to compare big bore solids to big bore solids.

The only numbers I find useful in the realm of terminal ballistics against personnel are:

Average weight of fragments,
Average number of fragments,
Average fragment velocity, and
Fragmentation pattern

lysanderxiii said:

The only numbers I find useful in the realm of terminal ballistics against personnel are:

Average weight of fragments,
Average number of fragments,
Average fragment velocity, and
Fragmentation pattern

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What about overpressure!?

Roverguy said:

An interesting article. Discusses the difference between sectional density and its importance to external ballistics and terminal sectional density, and its importance to penetration. Not sure that the term “terminal sectional density” really makes sense to me. It’s about terminal performance. And I think most experienced hunters, whether consciously or subconsciously, do make a calculation about sectional density and terminal performance. It’s been a long time since I’ve bought manufactured ammunition, but I would imagine that in hunting ammo, manufacturers generally do a good job of matching ballistic performance and terminal performance, no?

In any event, the assertion in the opening paragraph that sectional density doesn’t really matter seems gratuitously provocative. On the other hand, it will be music to the ears of the 6.5CM haters

“We need to talk about the sectional density (SD) of an unfired bullet. Why? Because you’ve been led to believe that it matters. And it doesn’t really. Not for most of us. I know this may come as a shock, given that many shooters believe that the higher a bullet’s initial SD, the deeper it will penetrate. Bullet catalogs list unfired SD as an important specification, after all, and the supposed correlation of SD and bullet penetration is all over Internet chat forums and even articles like this one. But if you just think about a bullet made of cheese, you’ll see what I’m getting at.”


https://www.fieldandstream.com/guns...vKOcFHPWbxoiCOFA1z7z5aUnCcvO9_eF-L_8Cy2Dn1ahk

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An article devoted to a false premise. That being that shooters choose bullets for penetration by sectional density. Nobody thinks like that. False
premise clickbait.
It is bullet construction that is primary.

mcb said:

If sectional density was useful then if I give you the following data you should be able to tell me which pernitrates better.

Bullet 1: SD = .192
Bullet 2: SD = .242

But you can't without me providing significantly more data. Even if I gave you the weight and diameter (the minimum data needed to calculate the SD) of Bullet 1 and 2 you still would not be able to tell me which would pernitrate better.

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This is ridiculous. SD is one of many factors that matter. Which rifle is better? Well, if I tell you rifle 1 weighs 7.5 lbs and rifle 2 weights 8 lbs can you tell me which is better? No? Then rifle weight isn't a useful metric and doesn't matter when you evaluate a rifle.

35 Whelen said:

The two factors that determine the sectional density (SD) of a projectile are weight and diameter of the projectile. Change either of these and the SD changes.

Using the TSX pictured above as an example, it's SD as an unexpanded .358 caliber, 225 gr. projectile is .251. However, once it is fired into an animal and the bullet has expanded, the diameter is no longer .35", and in the case of the bullet removed from the bull elk, it is a (roughly) .75 caliber, 225 gr. projectile with a SD of .057.
Barsness succinctly pointed this out in his article (I'll try to find the article) with the intent of showing that the bullets that produce big, wide pretty mushrooms (large frontal areas i.e. greatly increased bullet diameter)penetrate less than those whose expansion results in a smaller mushroom (smaller frontal area, i.e. marginally increased bullet diameter)

It doesn't mean that one controlled expansion bullet is better than another, rather it shows that a hunter should do his homework before selecting a hunting bullet.

35W

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Yes, I understand that. There are many factors that affect how well a bullet penetrates. Starting sectional density is one. Final sectional density (a factor of final weight and final frontal diameter) is another. I was just pointing out a simplified comparison that provides some information. It is far from perfect. If you really wanted to analyze this you would have to use calculus since we are interested in how far the bullet penetrates and the SD is a function of how far the bullet has penetrated into the animal (because it mushrooms and looses mass as it penetrates).

I would expect that engineers at Nosler, Barnes, Hornady, etc. have a very sophisticated model of bullet penetration that includes bullet mass, bullet diameter, material properties (hardness and ductility of the jacket; hardness and ductility of the core), etc.

mcb said:

What about overpressure!?

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Normally, you tailor to optimize the fragmentation, and whatever pressure effects you have, are what you get.

wombat13 said:

This is ridiculous. SD is one of many factors that matter. Which rifle is better? Well, if I tell you rifle 1 weighs 7.5 lbs and rifle 2 weights 8 lbs can you tell me which is better? No? Then rifle weight isn't a useful metric and doesn't matter when you evaluate a rifle.

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Exactly my point. SD in a vacuum tells us very little. BUT this is true of nearly all the numbers we bandy about in these thread. Kinetic Energy, Momentum, Power Factor, SD, TKO, HITS, OGW, LI etc. Without the data used to calculate these values the values themselves, in isolation, are nearly useless. And even if we have the numbers to calculate all these values (mass, velocity, caliber will allow us to calculate all the above quantities) we still lack critical information about the bullet construction.

My driver’s license lists height, weight, eye color, hair color, address, state of residence, date of birth, and a unique identifying number…

We don’t live and die by one number. This isn’t so complicated. No sense in being purposefully obtuse.

Hugger-4641 said:

That still doesn't equate to more lethality if it punches thru instead of expanding.

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Exactly.

I'm a numbers person. But I pay little attention to SD when choosing bullets for hunting applications. I could not tell you the SD of the different 100-120 gr .257 bullets used in my .25-06 or the 130-150 gr bullets I use in the 7mm-08 for mule deer. Don't care. What I do care about is that they will penetrate through and through with a broadside shot, but do a lot of expanding on their way. And I choose bonded bullets with heavier jackets for bigger critters like Elk, where I will also often opt for the 8mm Rem Mag instead of the smaller rounds, particularly if I'm after bulls. Again, no idea the SD of the 200 gr. Accubonds and 220 Gr. Gamekings I use in that rifle, I just know they do their job well.

MachIVshooter said:

Exactly.

I'm a numbers person. But I pay little attention to SD when choosing bullets for hunting applications. I could not tell you the SD of the different 100-120 gr .257 bullets used in my .25-06 or the 130-150 gr bullets I use in the 7mm-08 for mule deer. Don't care. What I do care about is that they will penetrate through and through with a broadside shot, but do a lot of expanding on their way. And I choose bonded bullets with heavier jackets for bigger critters like Elk, where I will also often opt for the 8mm Rem Mag instead of the smaller rounds, particularly if I'm after bulls. Again, no idea the SD of the 200 gr. Accubonds and 220 Gr. Gamekings I use in that rifle, I just know they do their job well.

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So you use 100-120 grain bullets in .25-06, 130-150 grain bullets in 7mm, and 200-220 grain bullets in 8mm? Why? If only there was a number that would capture why we typically use a heavier bullet as we increase caliber.

Sectional density used to be a more important figure of merit than it is now, what with all the trick "controlled expansion" bullets available.

wombat13 said:

So you use 100-120 grain bullets in .25-06, 130-150 grain bullets in 7mm, and 200-220 grain bullets in 8mm? Why? If only there was a number that would capture why we typically use a heavier bullet as we increase caliber.

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Exactly! If you're using heavy-for-caliber bullets for larger game, you're still paying attention to SD, even if you ignore the number that quantifies it. In a given caliber, everyone is going to choose a heavier-for-caliber bullet for moose than they would for prairie dogs. SD just quantifies 'how heavy' for its caliber.

Okay, while it is true, higher sectional density equal better penetration performance, it is not a magic number in terminal performance.

A 1/4" diameter by 3 inch long steel flechette (approx 575 gr) is going to have a SD over 1.0, but I don't think it would be a good hunting round. And, I think a 575 grain .50 cal bullet at a similar velocity would be more effective.

You're Overthinking It Again !
As long as the bullet size starts with a 4 ... and the weight is at least 220 grains ...
It will do the job nicely .
Gary

lysanderxiii said:

Okay, while it is true, higher sectional density equal better penetration performance, it is not a magic number in terminal performance.

A 1/4" diameter by 3 inch long steel flechette (approx 575 gr) is going to have a SD over 1.0, but I don't think it would be a good hunting round. And, I think a 575 grain .50 cal bullet at a similar velocity would be more effective.

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Nobody said it was a magic number. There are no magic numbers. It is but one factor, of many. Far from the only.

wombat13 said:

So you use 100-120 grain bullets in .25-06, 130-150 grain bullets in 7mm, and 200-220 grain bullets in 8mm? Why? If only there was a number that would capture why we typically use a heavier bullet as we increase caliber.

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I didn't say it doesn't matter, I said I don't pay much attention to it, because it really doesn't tell you anything about how the bullet will perform on living flesh, whether it will explode when it hits a rib or punch an ice pick hole through the animal with zero expansion.

Ergo, for hunting, I run midweight to heavy for caliber bullets with appropriate construction for the game to be taken. The SD is what it is.

Adding the word "terminal" is superfluous.

Goosey said:

Adding the word "terminal" is superfluous.

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How is it superfluous? The original subject is comparing the sectional density of the bullet in it original form and in it's form post terminal impact. It's weight/mass may change and it's effective diameter will also likely change.

Goosey said:

Adding the word "terminal" is superfluous.

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Don't be talking dirty ...

Jim Watson said:

Sectional density used to be a more important figure of merit than it is now, what with all the trick "controlled expansion" bullets available.

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I’m not so certain I think “controlled expansion” means a damned thing.

Bonded core, segmented core, monometal, gilding metal alloy, etc - these mean things. “Controlled expansion” seems to mean “maybe a thicker jacket than some of our other bullets, which might or might not have thicker jackets than some other brands”.