I continually see comments on this forum about sectional density being some kind of a predictor of terminal bullet performance. Such as a 140g 6.5 bullet providing better penetration than a 140g .277 bullet, and therefore being superior on big game. As the SD is no more than weight divided by the square of the diameter, I can grasp how a non-deforming solid of minimally higher SD would out-penetrate a bullet with a lesser SD...in theory. But since the vast majority of hunting appropriate bullets expand in a manner much more determined by what they hit than what they looked like getting there, how can small differences in SD even be quantified in terms of actual penetration? Seems to me the unpredictable differences in expanded diameter when contacting hide/meat/bone, especially when squared, instantly obviate any theoretical advantage the higher SD bullet might have had. Sure, I can see how a doubling of SD would predict better penetration, all other things being equal, but a few ticks three places to the right of the decimal point? Has anyone demonstrated - in a repeatable manner - that slight increases in SD, with equal expanding bullets, equate to more penetration IN something? At a given velocity and bullet construction, I'd guess bullet weight means more to penetration than SD.
Sectional density - help me understand
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Alaskan Ironworker
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You need to get out and go hunting friend, lol. You’re hurting my brain.
A bullets performance on game is not a predictable event. Shot placement and bullet construction are the most important factors for a successful kill. A high SD, in theory, will more effectively retain momentum and should result in greater penetration. But that is only theory. Hit a bone with a soft bullet and it doesn't matter.
Sometimes we get too hung up on statistics.
Sometimes we get too hung up on statistics.
rabid wombat
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This all ties back to all things being equal...they are not....
What sounds like a better elk round? A .300 win mag running a 150 gr bullet? 165? 180? 220?
.270 running 130 gr? 140? 150? 160? 180?
If I stood that Uber long 180 .270 next to the not nearly so long .30 180 bullet would you lean towards the .270? If it meant 10(ish I'm not doing this much math tonight) ftlbs less recoil? What about the .300 220 vs a .338 210? Does 10 grains of weight really matter as much? Or do you KNOW that that 220 is a HEAVY bullet and the 210 is not in the same class?
Riomouse911
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Factory ammunition and reloading component companies are really, REALLY good at giving us perfect-condition, laboratory-achieved numbers for their products to contrast, compare and then argue over...originally around the campfire, then at the liars bench at our local gun stores and now at our computer keyboards.
Everything you asked about in the question you posted has so many variables to consider I don't think Deep Blue could possibly have the computing power to take everything into account to answer it.
When it comes to performance on game the above post was correct; sufficient energy, bullet construction and shot placement will win the day whether the SD of the bullet that whacks the buck's shoulder is .200 or .500.
Don't get mired down in the minutiae; it's a deep, thick morass that you'll rarely have enough energy to climb out of.
Stay safe.
Everything you asked about in the question you posted has so many variables to consider I don't think Deep Blue could possibly have the computing power to take everything into account to answer it.
When it comes to performance on game the above post was correct; sufficient energy, bullet construction and shot placement will win the day whether the SD of the bullet that whacks the buck's shoulder is .200 or .500.
Don't get mired down in the minutiae; it's a deep, thick morass that you'll rarely have enough energy to climb out of.
Stay safe.
Varminterror
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This is why your theory is broken.
DocRock
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It's a variable. One among many. When someone tells you that the 6.5 mm 140 grs has better SD than a 180 grs .308, so the 6.5 CM is a better choice for elk than the 30-06, smile politely and move on...
Sectional density is a very good predictor of penetration.
A picture is worth 1000 words. Top bullet is longer in relation to diameter (higher SD) and assuming equal construction will penetrate deeper than the bottom bullet. It doesn't matter what the caliber or bullet weight.
Comparing a 143 gr 6.5 bullet to a 178 gr .308 bullet. Both have very similar SD's, same construction, penetration is virtually the same. The 308 does more damage initially, but at the end of the day any animal hit with either bullet is going to be dead. This isn't a new concept. It's been well documented for over 100 years and why rounds like the 6.5X55 and 7X57 using high SD 160 and 175 gr bullets were so popular for hunting African game since the 1890's. All African game, even elephant.
A picture is worth 1000 words. Top bullet is longer in relation to diameter (higher SD) and assuming equal construction will penetrate deeper than the bottom bullet. It doesn't matter what the caliber or bullet weight.
Comparing a 143 gr 6.5 bullet to a 178 gr .308 bullet. Both have very similar SD's, same construction, penetration is virtually the same. The 308 does more damage initially, but at the end of the day any animal hit with either bullet is going to be dead. This isn't a new concept. It's been well documented for over 100 years and why rounds like the 6.5X55 and 7X57 using high SD 160 and 175 gr bullets were so popular for hunting African game since the 1890's. All African game, even elephant.
edwardware
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You grasp the concept of SD's contribution to penetration, but it sounds like you're stuck behind the variability of expansion and hide/bone/flesh, etc.
The fact that the unpredictable variation (expansion, hide/bone/flesh) is large doesn't imply that the effects of smaller measurable factors disappears, just that the measurable factors aren't enough to predict performance in any individual case.
So, an SD+0.1 bullet won't necessarily kill the first moose you shoot with it any faster than the SD-0.1 bullet you used last year, but across 1000 hits, the SD+0.1 bullet will penetrate better.
mcb
Member
With a good hunting bullet sectional density changes at impact. As the bullet expands sectional density drops dramatically.
In a hunting situation with expanding bullets and living targets there is no one single number that predicts much of anything reliably.
In a hunting situation with expanding bullets and living targets there is no one single number that predicts much of anything reliably.
Chuck R.
Member
But using your same picture:
What relevance would the higher SD have IF, the lower bullet was a solid, and the upper an expanding?
I believe that bullet construction mitigates the value of SD to some extent.
Under the parameters of equal bullet construction, that long one could shrink too.But using your same picture:
What relevance would the higher SD have IF, the lower bullet was a solid, and the upper an expanding?
I believe that bullet construction mitigates the value of SD to some extent.
daniel craig
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I’m no smart person but I had it explained to me like this, what penetrates a piece of cloth better, a needle or a ballpoint pen?
Longer, thinner bullets will penetrate better but then you need appropriate mads and velocity.
Longer, thinner bullets will penetrate better but then you need appropriate mads and velocity.
I think as long as no one gets too excited over the term, both for and against, its serves some purpose. Its ability to predict a neat figure for penetration ends with penetration in air where it is one of the variables in the ballistic coefficient formula. Anything more than that the math gets tricky so its better just using it as a general comparison tool. When I hear or see SD I dont attribute too much too it apart from a basic comparison between bullets in calibres. You can at least use it to work out what is a 'long for calibre' or 'short for calibre' bullet and avoid extremes at either end.
For example I hear of a guy using a 250 grain bullet in 30-06 who doesnt know better for deer, I would say "buddy that weight or SD is probably overkill. Save yourself some recoil and trajectory and get a 180 grain" I can do this with complete honesty and not worry I have offended the ghosts of dead physicists.
For example I hear of a guy using a 250 grain bullet in 30-06 who doesnt know better for deer, I would say "buddy that weight or SD is probably overkill. Save yourself some recoil and trajectory and get a 180 grain" I can do this with complete honesty and not worry I have offended the ghosts of dead physicists.
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mcb
Member
Except in your example the needle and ballpoint pen do not expand when they encountering the cloth. A good hunting bullet expands and the instant it expands it's section density goes down. The sectional density of a bullet in its terminal phase is changing its sectional nearly continuously so how can its SD in flight (pre-impact) have any meaningful bearing on its terminal performance?
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earlthegoat2
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I used to be a hard acolyte of SD and over time I have come to taking a broader approach on bullet performance.
I still value SD highly but as I started using straight walled rifle rounds for Midwest hunting which typically have lower SDs, I have come to appreciate its diminishing importance as bullet weight increases across all bullet diameters. Namely, larger bullet diameters. Also, as the game hunted increases in size and toughness, SD can once again, become increasingly important.
SD is just another number to analyze. There are other numbers out there too. Taylor KO value and energy ft-lbs for example however flawed they are, they still give some information just like SD. Also, bullet construction is, IMO, more important then SD. Much more.
Solids or FMJs of high SD also don’t guarantee penetration. If they hit something hard like a thick bone, they can and will tumble and even deform under the right circumstances. Sometimes brute power is still the determining factor in putting an animal down right now and watching it hop over the property line or into the thick swamp.
Since most of us are strictly deer hunters, the importance of SD is not that great. Many states have enacted laws of types and specs of firearms and ammunition (usually “expanding” that can be used for specific game and the hunter just really needs to be educated on bullet types, so a varmint bullet is not used for deer, and then they need to know shot placement.
I really don’t even care to bring up points directly or indirectly relating to sectional density unless someone is hunting larger and more dangerous game. Bullet construction becomes just as important even then.
I still value SD highly but as I started using straight walled rifle rounds for Midwest hunting which typically have lower SDs, I have come to appreciate its diminishing importance as bullet weight increases across all bullet diameters. Namely, larger bullet diameters. Also, as the game hunted increases in size and toughness, SD can once again, become increasingly important.
SD is just another number to analyze. There are other numbers out there too. Taylor KO value and energy ft-lbs for example however flawed they are, they still give some information just like SD. Also, bullet construction is, IMO, more important then SD. Much more.
Solids or FMJs of high SD also don’t guarantee penetration. If they hit something hard like a thick bone, they can and will tumble and even deform under the right circumstances. Sometimes brute power is still the determining factor in putting an animal down right now and watching it hop over the property line or into the thick swamp.
Since most of us are strictly deer hunters, the importance of SD is not that great. Many states have enacted laws of types and specs of firearms and ammunition (usually “expanding” that can be used for specific game and the hunter just really needs to be educated on bullet types, so a varmint bullet is not used for deer, and then they need to know shot placement.
I really don’t even care to bring up points directly or indirectly relating to sectional density unless someone is hunting larger and more dangerous game. Bullet construction becomes just as important even then.
Varminterror
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Paraphrasing, less crudely, a line my dad used to say in times like this:
“What if my aunt was born a man? She’d be my uncle.”
Bullet construction doesn’t truly mitigate the value of SD, and we have seen that it does not in application, save for the most extreme “solid vs. HP” where the comparison in performance is nearly an absurdism.
When you appropriately couple variables of bullet construction, SD (which is mass and diameter, two independent variables rolled into one), and impact velocity, you retain a very good predictor for penetration and killing potential. Almost like a multivariate slide-rule.
In fairness, this isn’t quite apt, as common bullets used in big bore rifles for big game don’t have low SD’s, despite their blunt appearance.
A 400 grain slug from a 45-70 has an SD of .272. Comparatively, this matches a 180 grain bullet from a 30-06, matches a 250 grain slug from a 338 win mag, exceeds the SD of a 140 grain 7-08 slug (.248), and exceeds that of a 105 grain slug in a 6 creed (.254). A 500 grain 50 caliber slug from a 505 Gibbs has an SD of .280.
This certainly isn’t what most folks would call “extremely low SD,” considering these 45 and 50 caliber bullets exceed the SD benchmarks held by the heaviest bullet used in many common respective calibers and cartridges. These big punkin’ balls may have low BC’s, but not low SD’s, certainly not extremely low.
mcb said:
I agree with you on this. I focus on controlling the variables that I can such as choosing a monolithic copper hunting bullet to maintain as close to 100% mass as possible. Shot placement and velocity are also variables that I have some control over. SD is not a constant, and not even close to a constant for some bullets, e.g. a cup and core ELD-X. Choose calibers, bullets and velocities that have empirically demonstrated to be effective on the type of animal you're hunting.
daniel craig
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CraigC
Sixgun Nut
Sectional density is an important factor but it is one of many. People like simple answers but terminal ballistics is far too complicated variable for a simple answer like "SD" or "TKO" or "kinetic energy" to be of any use. SD is really best used as a tool not so much to predict penetration but to compare one bullet to another. To compare a known to an unknown. One can reasonably assume that bullets with comparable construction, impact velocity and sectional density will behave similarly on game but there are no absolutes.
All-copper bullets don't diminish the importance of sectional density, weight relative to diameter, they only move the scale. A 300gr copper solid will still outpenetrate a 200gr solid of the same diameter and shape.
All-copper bullets don't diminish the importance of sectional density, weight relative to diameter, they only move the scale. A 300gr copper solid will still outpenetrate a 200gr solid of the same diameter and shape.
I agree with you. What most people forget is your statement about similar bullet construction.
If you run a 150 bonded bullet and a 180 ballistic tip. The bonded will most likely out penetrate it. But focusing on SD only says it wouldn't.
mcb
Member
Right. SD alone tells you almost nothing. SD for two different weight bullet of similar construction and same caliber still does not tell you much more since you still don't know what launched it. I suspect a 150gr expanding copper bullet with a SD of .225 is going to out penetrate a 220gr expanding copper bullet with a sectional density of .331 IF the 150gr is launched from a 300 RUM and the 220 gr from a 300 BO. The velocity and resulting momentum/kinetic-energy (two inseparable values) cannot be ignored either.
I would argue few of us use just one number despite that one number, whatever it is, we like to spout off about. We always do so in some assumed context (a bunch of other numbers) even if we don't consciously acknowledge the limited context we are arguing from. You often see many say a good deer hunting bullet has a SC of at least .2 and they never bother to say that they are talking about rifle cartridges shooting above roughly 2200 fps from medium calibers (.243 - .45). The typical 1 oz 12 gauge slug only has a SD of .117 and yet anyone that has shot medium or big game with it knows how devastating they are terminally. But the 12 gauge slug lies way outside all of the other parameters of the guys claiming SD of .2 or higher is a good guide line.
Without context SD (or most of the other single number quantities we love) are pointless. In a narrow context SD has some meaning but then your not using just one number are you.
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