Sectional Density vs Terminal Sectional Density

[Roverguy]

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

 

[Varminterror]

Over a hundred years of in-field provenance and hundreds of years of physical science disagree with the pandering article by F&S… just another attempt of print media to replicate clickbait and garner fleeting attention as their star fades into nothingness…

 

[Chuck R.]

I've posted that article before here. I don't look at it as SD doesn't matter so much as it doesn't matter as much as it once did, due to changes in bullet construction.

A case in point, monolithic bullets often penetrate better than their cup and core counterparts, even though the cup and core bullet may have a higher SD. A Nosler .308 180 grn AB has the same SD as a .308 180 grn BT. If relying on just SD, they should penetrate equally, but in this case the bullets construction plays a more important role in it's performance than the equal SD number.

SD is really a pretty simple calculation:

The formula for calculating sectional density is pretty simple and straight forward. Take the bullet weight and divide by 7000. This number is then divided by the bullet diameter squared. Two bullets of equal weight and the same diameter will have equal sectional sectional density.

But as you can see, the above doesn't take into account the bullet construction. I think that's the gist of what the author is trying to get at.

Here's another:

https://bulletin.accurateshooter.com/2021/06/sectional-density-of-bullets-what-you-need-to-know/

The next time you’re choosing a bullet, look at the construction and less at the sectional density number. It’s all about the construction anyway. If you have any questions or would like to discuss sectional density or bullet penetration further, please give us a call at 800-223-8799 or shoot us an email at [email protected].

BTW the above article pretty much agrees with the OPS article, just less "drama" and was written by a Sierra technician.

 

[mcb]

I agree unfired SD has very little meaning to terminal performance with the exception of bullets designed not to expand, like armor piercing and similar. Any good hunting bullet has a dramatic change of it SD, to the lower, after impact.

If you're at all curious I did a very subjective terminal testing with a variety of weapons on armadillos and of all the "numbers" I calculated (Kinetic Energy, TKO, SD (unfired) etc). The only one of the numbers calculated that did not correlate with my experience of their terminal performance was SD.

https://www.thehighroad.org/index.p...e-soft-on-the-inside-ballistic-target.913245/

It's nice to see an article supporting that idea. I have actually met Richard Mann at a shooting event in Texas. A little gruff but personable enough and told quiet a number of interesting stories.

 

[Varminterror]

We don’t know anything about expansion if we don’t know impact velocity, construction, and substrate.

Run a bullet into bone and it opens differently than if it runs into ribs. Run a thicker jacket into the same spot as a thinner, one opens more. Put 500 yards of air under a bullet from a 308 instead of 60 yards from a 300wm, and penetration will change a bunch…

SD is a number we have. Construction is also information we have. Impact velocity is information we can predict.

Make the bet for yourself - two bullets of the same construction run into the same substrate at the same speed, one is a 110 grain 30 cal and one is a 110 grain 6mm. Which one penetrates farther?

 

[tark]

F&S is a publication I put in the same category as Consumer Reports. Jack of all trades but master of none. I would like to ask the authors of that article what are the four phases of ballistics. Everybody knows interior, exterior and terminal, but there is a fourth.

 

[mcb]

Couple random additional thoughts.

In the engineering/physics disciplines, especially those dealing with fluid (gas or liquid) dynamics sectional density has a slightly different definition. It seem the commercial firearms community simplifies it a bit.

Engineers define sectional density as: SD = m/A
m = mass
A = frontal area (area presented perpendicular to the fluid flow)

Ballistic simplification: SD = m/D^2
m = mass if you doing metric but weigh if you using US customary units.
D = diameter (D^2 is proportional to area)

The units and values vary but as long as we are talking about round front areas they remain proportional to each other. If you double the SD in engineering units you will double the SD as calculated in ballistics (again assuming round frontal areas and we remain on earth). That said I think this difference in definition become important when we start looking at the TSD (Terminal Sectional Density). The diameter squared of an expanded bullet in some case captures a proportional representation of the frontal area well enough in other case not so much.


Using the diameter squared of this expand 38 Special would have minimal error in the frontal area calculation.


Using the expanded diameter of this bullet has a fairly substantial error compared to the actual calculated area.


The diameter of this expanded projectile has a huge amount of error compared to calculating the actual frontal area

We also are making the assumption our bullet don't tumble in the target.

 

[Hugger-4641]

Make the bet for yourself - two bullets of the same construction run into the same substrate at the same speed, one is a 110 grain 30 cal and one is a 110 grain 6mm. Which one penetrates farther?

Well, I'm an automation engineer, not a ballistic engineer, but my money is on the 6mm penetrating further. Less frontal area = less resistance = slower deceleration.
That still doesn't equate to more lethality if it punches thru instead of expanding.

 

[jmr40]

Like most things there are no simple answers. I've never heard of anyone trying to measure terminal sectional density. But the sectional density of an unfired bullet can be a pretty good indicator of how much penetration you will get when a bullet impacts game relative to another bullet with a lower SD.

And this has been proven pretty reliable for over 100 years. And this isn't just mathematical theory. It has been proven time and again by shooting bullets into various mediums as well as into game.

But you have to eliminate as many other factors as possible to ensure the only thing that changes is the bullets SD. You have to be using bullets of the same construction.

For example, if both bullets are Nosler Ballistic Tips and both impact at roughly the same speed, and if they hit the same spot on a game animal, and if they are close to the same weight, the higher SD bullet will almost always penetrate deeper.

If one bullet impacts significantly faster, it will expand more and penetrate less regardless of SD. If one hits bone, and the other does not there will be a difference. If one is significantly heavier. If one bullet is a Ballistic Tip, and the other is a Partition, the Partition will likely penetrate more regardless of SD. The more factors you change, the less accurate SD becomes.

But it can matter, especially when taken to extremes. The Marlin 444 was initially a failure because the 240 gr bullets available proved to be poor penetrators. The 45-70 was revived to be able to shoot 350-500 gr bullets with much higher SD's.

 

[Hugger-4641]

I definitely agree with construction being more important. Quality is also important. Theory and specifications go out the window when a bullet like the Core loct has a good design but poor QC and doesn't do what it's supposed to do on paper.

 

[mcb]

Like most things there are no simple answers. I've never heard of anyone trying to measure terminal sectional density. But the sectional density of an unfired bullet can be a pretty good indicator of how much penetration you will get when a bullet impacts game relative to another bullet with a lower SD.

And this has been proven pretty reliable for over 100 years. And this isn't just mathematical theory. It has been proven time and again by shooting bullets into various mediums as well as into game.

But you have to eliminate as many other factors as possible to ensure the only thing that changes is the bullets SD. You have to be using bullets of the same construction.

For example, if both bullets are Nosler Ballistic Tips and both impact at roughly the same speed, and if they hit the same spot on a game animal, and if they are close to the same weight, the higher SD bullet will almost always penetrate deeper.

If one bullet impacts significantly faster, it will expand more and penetrate less regardless of SD. If one hits bone, and the other does not there will be a difference. If one is significantly heavier. If one bullet is a Ballistic Tip, and the other is a Partition, the Partition will likely penetrate more regardless of SD. The more factors you change, the less accurate SD becomes.

But it can matter, especially when taken to extremes. The Marlin 444 was initially a failure because the 240 gr bullets available proved to be poor penetrators. The 45-70 was revived to be able to shoot 350-500 gr bullets with much higher SD's.

You just put so many qualification on your use of SD that one must ponder if it actually tells you anything more than you already new from all the factors you are qualifying it with.

In my experience SD alone has no correlation to pentation.

 

[earlthegoat2]

The only times I have heard vehement arguments toward sectional density have been when solids were the bullet of consequence….or someone who knew enough to be dangerous was arguing.

Either way, I threw sectional density out the window when I quit reading Chuck Hawks articles. Haha. That guy loved sectional density.

The more I read about theoretical and mathematical ballistics the more I tend to just grab a bigger gun.

 

[mcb]

F&S is a publication I put in the same category as Consumer Reports. Jack of all trades but master of none. I would like to ask the authors of that article what are the four phases of ballistics. Everybody knows interior, exterior and terminal, but there is a fourth.

I'll bite, what is the fourth phase of ballistics? The only sort of interesting thing I can think of it the transition from internal to external. Some interesting things happen as the bullet exists the muzzle along with gas interaction with various muzzle configurations.

 

[tark]

I'll bite, what is the fourth phase of ballistics? The only sort of interesting thing I can think of it the transition from internal to external. Some interesting things happen as the bullet exists the muzzle along with gas interaction with various muzzle configurations.

Internal, transitional, external and terminal. The transitional phase begins the instant the base of the bullet clears the muzzle and ends when the expanding gasses behind it have ceased to push the bullet. That's a crude way of putting it.

The transitional phase is the shortest and briefest of the four, for obvious reasons. It only lasts a few milliseconds. It is studied for the exact reasons you mentioned. Designing things like flash suppressors and muzzle brakes involve much study of the transitional phase.

I know little about ballistics. My son has a friend who knows...ahh...a whole lot! That's where I get my info. Malcom has a double doctorate in thermodynamics and plasma physics. Since thermodynamics and ballistics are litter mates, I have learned a lot from Malcom. That other doctorate is pretty much useless unless you are designing and building nuclear warheads for the DOD , which is what Malcom did when he was working at Lawrence Livermore National Laboratories.

 

[CraigC]

Sectional density absolutely matters. Monolithics don't change its importance, they just move the scale back.

 

[35 Whelen]

Way back when we hunters mostly used cup and core bullets, sectional density gave us a rough idea of how bullets could be expected to penetrate given similar design, impact velocity, etc. But changes in bullet designs have altered those expectations.
I've been tracing bullet paths and digging bullets out of game, when those bullets managed to stay in the animal in one piece, since I was a teenager. My favorite example of how bullet design influences penetration more than sectional density comes from two bullets fired (ironically) from the same cartridge (35 Whelen), fired by the same person (my father), into the same game (bull elk) with the two bullets impact velocities being within just a few feet per second of each other.
The first example is a Barnes .358" 225 gr. TSX with a SD of .251 that struck the south end of a northbound bull with an impact velocity at 100 yds. of roughly 2420 fps. It penetrated from said south end to in front of the right shoulder where it stopped under the hide. It's retained weight was 225 gr. (100%).



The second example is a Speer .358" 250 gr. SP with a SD of .279. I thought I'd do ol' dad a favor and work up a "heavy" bullet load for his Remington 700 Classic for him and happened to have a box of these bullets I'd gotten in trade. This bullet was fired into a bull broadside at a quartering-away angle with an impact velocity at 45 yds. of roughly 2450 fps. It broke two ribs where it struck then penetrated to the off shoulder where it stopped. The blood trail and internal damage was like nothing I've ever seen. I don't recall its exact retained weight, but I want to say it was less than 100 gr. (<40%).



So, this would be an extreme example of how bullet construction has more of an influence on bullet penetration, in expanding bullets, than does sectional density.



An example of how SD being effectively decreased because of expansion-

Several years ago I began casting hollowpoints for my revolver cartridges to use for deer and hog hunting. I popped a small buck at 48 yds. with a Uberti Bisley 45 Colt loaded with a 265 gr. SWCHP at 1040 fps MV. The bullet hit somewhat high in the back, went through the backstraps , clipping a vertebrae and came to rest under the hide on the offside.



Fast forward a few years, same cartridge, same bullet only in solid form and about 50 fps slower, and another similar buck shot broadside at a bit over 40 yds. That bullet is laying somewhere in my pasture as it penetrated completely.

A .430" 250-ish gr. cast HP fired from a .44 special at 1105 fps MV into a small buck at 35 yds. broadside where it stopped under the skin in the off-side shoulder.



Same caliber/weight cast bullet, only in solid form, that struck a large buck at similar velocity behind the last rib on the left side and penetrated to the juncture of the right shoulder and the neck.



John Barsness of Handloader and Rifle magazines beat Richard Mann to the punch as he wrote an article on this very subject probably 10-12 year ago. The premise is pretty simple; given bullets of the same caliber, weight and impact velocity, the smaller the frontal area after expansion, the greater the penetration. As such, a bullet such as a Barnes TSX with its small frontal area post-expansion, can be expected to out-penetrate a heavy jacketed, bonded core bullet such as Swift Scirocco or Nosler Accubond with their large frontal areas post-expansion.

35W

 

[mcb]

As with most of these arguments one number alone tells you very little. And very few of us use one number as you can see by how many other numbers people use to qualify that SD has meaning. For SD to be useful many argue you need to know something about velocity and construction. At that point I can calculate any one of the other numbers we like to bandy about in these threads.

Mass, velocity, caliber (you could argue caliber is part of construction) and construction covers all your bases and your can calculate nearly all the favorite magic numbers from the first three. Kinetic energy, Momentum, Sectional Decision. Taylor KnockOut. Hornady Index of Terminal Standards. Optional Game Weight, Lethality Index, etc. But all of those fancy numbers still come back to mass, velocity, caliber and none of them capture the critical construction. They are also all mathematical depended on each other. You can't change one without changing the others in all but a few exceptions. ie Sectional Density fails to capture velocity. Kinetic Energy fails to capture caliber.

The numbers are nice especially if you know where they come from and the math behind them. They still sit secondary to going out and shooting **** with them and seeing for yourself.

 

[Demi-human]

The Fifth Phase Of Ballistics:
Luck!

 

[mcb]

The Fifth Phase Of Ballistics:
Luck!

 

[Howa 9700]

When it comes to figuring out the best way to kill an animal, I'm coming around to this guy's way of thinking:

https://www.ballisticstudies.com/

His stuff on wound research is pretty helpful. My take........it's a 3 legged stool. Ideal shot placement, enough velocity to generate enough hydrostatic shock to put animal into instant coma and a bullet that expands at correct velocity to create a massive wound channel thru the lungs to bleed him out before he wakes up.

Do that and animal drops where he stands and never moves.

Many facets to that gem, but that is the gist of it.

The other takeaway.....at least for me......is that most hunters have no business trying to shoot at animals beyond 500 yards.....for a variety of reasons.

 

[wombat13]

Of course, sectional density matters when it comes to penetration. Bullet construction matters in that it changes how the sectional density of the bullet changes as it passes through the animal. One would really need to use calculus, but a simplification is that the bullet with the higher average sectional density as it passes through the deer would be expected to penetrate further. Even simpler is to compare the expected final sectional density of two bullets. For example, as Barnes TTSX typically retain very close to 100% of the initial weight (only losing the plastic tip) therefore the final sectional density is the same as the starting SD. Nosler Partitions are designed to retain 65-70% of the initial weight, so the final sectional density is about 70% of starting sectional density. Admittedly, this is quite simplified because, as posted above, the frontal area of the bullets will likely be different.

So just as very quick, crude comparison when considering expected penetration and bullet weight for TTSX bullet I divide the TTSX weight by 0.7 and I expect to get penetration similar to a controlled expansion lead bullet of that resulting weight. I shoot a 150 gr TTSX in my .300WM. I expect it to penetrate similar to a 214 gr Partition, that is very, very well. I'm shooting a 100 gr TTSX in my .25-06 and I would expect that to penetrate like a 143 gr partition. I also typically run the numbers using 0.8 for a more conservative estimate. Using that number the .308 150gr TTSX should penetrate like a 188 gr Partition and the .257 100 gr TTSX should penetrate like a 125 gr Partition.

 

[35 Whelen]

{snip}
For example, as Barnes TTSX typically retain very close to 100% of the initial weight (only losing the plastic tip) therefore the final sectional density is the same as the starting SD. {snip}

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

 

[mcb]

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.

 

[Roverguy]

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.

Point taken. But the article suggests “sectional density doesn’t really matter”. It does matter to ballistic performance, which has a bearing on terminal performance because if you don’t hit game in the vitals, terminal performance is somewhat moot. And sectional density has a major bearing on terminal velocity which will also has a bearing on penetration, again dependent on bullet construction.

So sectional density does matter to predictive penetration because it bears on ballistic performance and terminal velocity. But predictive penetration or terminal performance requires both knowledge of terminal velocity and bullet construction. And, even then, it’s just that: predictive.

 

[mcb]

Point taken. But the article suggests “sectional density doesn’t really matter”. It does matter to ballistic performance, which has a bearing on terminal performance because if you don’t hit game in the vitals, terminal performance is somewhat moot. And sectional density has a major bearing on terminal velocity which will also has a bearing on penetration, again dependent on bullet construction.

So sectional density does matter to predictive penetration because it bears on ballistic performance and terminal velocity. But predictive penetration or terminal performance requires both knowledge of terminal velocity and bullet construction. And, even then, it’s just that: predictive.

Yes for external ballistics SD also matters but in that context SD fails to capture all the needed data. For external ballistics we use Ballistic Coefficient (instead of SD) and its associated drag models. Sectional density is technically hidden down inside the models use by most ballistic calculators but we as users never touch that. We input Ballistic Coefficient and initial velocity.

ie A 30-caliber 150gr BTSP and a 150gr RNFB bullet both have the same Sectional Density (.226) but the BTSP has a BC of .349 (G1) but the RNFB has a BC of .186(G1) . So sure a high SD bullet is probably going to carry its velocity down range better than a low SD bullet but its not always true and BC explicitly let us predict exactly how much velocity we will have at any point down range. All we need is the initial velocity and the BC and we know it exactly (assuming our ballistic model G1, G7 etc) fits our selected bullet well over the used velocity range.