Bullet Sectional Density (SD) ... the most meaningless variable for the hunter!!

[MCMXI]

Sectional Density is a simple calculation being the ratio of the bullet's weight in lbs to its diameter in inches squared.

E.g. for a 127gr .264 cal bullet: 127gr * 1/(7000gr/lb )*1/(.264^2) = 0.260

It has units of PSI so it must mean something right? Kind of if you're trying to calculate a bullet's BC, but even then it's a dumb-downed variable to make the calculations manageable. A bullet in flight has an "angle of attack" that's not parallel to the direction of flight. For the hunter concerned with terminal ballistics, particularly bullet penetration, SD meaningless!! From the definition of SD, the value is only constant in the direction of the bullet's long axis and only for the bullet's original and unaltered geometry i.e. constant mass and constant diameter. Some on this forum (one in particular) throw SD around in the hunting context as if it has meaning. For the target shooter it's meaningless since BC absorbs a "close enough" value for SD. For the hunter it's at best simplistic and at worst erroneous. When a bullet contacts an animal does its diameter remain constant? Does its mass remain constant? Does it only move in a direction indicated by its long axis? In short, no. Bullets expand, bullets tumble, bullets lose mass, bullets deform. Comparing bullets for hunting based on SD is ridiculous and ignorant.

This is just my opinion based on hunting, retrieving bullets from gel tests/berms, and the destruction of a few brain cells.

 

[Chuck R.]

Well you're not alone in your thinking:

The temptation to compare the SD of two bullets of a different caliber in order to make an assumption about penetration or killing ability exists, and you might often hear this discussion around the counter at your local gun shop. The fact is, regardless of weight, differences in bullet construction, impact velocity and deformation progression, make SD irrelevant.

Further evidence that SD by itself is no indication of terminal ballistics is when you compare bullets of the same weight and caliber, but of different designs that impact at the same velocity. A test with six different 180-grain, .30-caliber bullets, from four different manufacturers, showed a penetration depth variance of as much as 27 percent. On the shallow end, a Swift Scirocco II penetrated 14.5 inches and on the deep end a Winchester XP3 penetrated 19.5 inches. Comparing the SD of two unfired bullets is nothing more than comparing the ratio of their weight to their diameter. It’s like comparing the body-fat percentage of two boxers in an effort to determine which one will hit the hardest or win the fight. As far as penetration is concerned, the only formula you need to know that involves SD, is that SD = BS, and that’s a formula anyone can compute, even without a calculator.

https://gundigest.com/gear-ammo/amm...od-measure-of-a-bullets-penetration-potential

 

[MCMXI]

Yep, and I bet dollars to donuts that the penetration tests mentioned didn't involve skin, hide, fur, bone or tendons, or anything else that affects expansion, tumbling etc.

It's surprising to me that ammunition and bullet manufacturers publish SD. Why bother? BC I get, bullet weight, bullet diameter and bullet length, sure. These are variables I can't measure looking at their website ... but SD? I can calculate that if I need it, which I don't. Smoke and mirrors perhaps.

 

[DocRock]

The 6.5 CM fanboy mafia is about to smack you upside the head with a CNN quality hit piece...

 

[MCMXI]

The 6.5 CM fanboy mafia is about to smack you upside the head with a CNN quality hit piece...

You're probably right!

 

[horsemen61]

Yep, and I bet dollars to donuts that the penetration tests mentioned didn't involve skin, hide, fur, bone or tendons, or anything else that affects expansion, tumbling etc.

It's surprising to me that ammunition and bullet manufacturers publish SD. Why bother? BC I get, bullet weight, bullet diameter and bullet length, sure. These are variables I can't measure looking at their website ... but SD? I can calculate that if I need it, which I don't. Smoke and mirrors perhaps.

I agree with this

 

[CraigC]

Could not disagree more. I think some have this weird idea that SD is touted as the answer to any question. It is not. It is merely one of many factors affecting penetration. Not to be taken on its own. That said, it probably matters less to expanding rifle cartridges than it does non-expanding solids.

 

[Random 8]

I think it can be useful, but one needs to compare apples to apples. Let's say your favorite Elk combo for many moons has been a 30-06 with 180 grain NPs, but your recent shoulder surgery won't handle that level of recoil anymore. Comparing a simple figure, you'll find the 6.5 140 or .270 150 grain versions of the same bullet at similar velocities to be in that wheelhouse and likely suitable, but the .30 cal 150 to have some issues in the SD department. Now, a 150 Barnes TSX may be suitable, but that's an orange.

Now most of keyboard warriors on here get much more advanced than that, but the average Joe craves simplicity. That's why many bullet companies put a picture of a Deer, a Coyote or an Elk next to specific bullets in their marketing.

 

[Llama Bob]

Sectional density along with its companion weight retention is the #1 determinant of how deeply a bullet of any given type penetrates. That matters to the hunter, full stop. OP's post is absurd nonsense.

And if your hunting bullets are tumbling, you've got a BIG problem.

 

[CraigC]

Here's how I boil it down. Does bullet weight matter at all? Do you believe that in general, heavier bullets penetrate more deeply than lighter ones? If you answer yes to either of those questions, SD matters.

 

[CarJunkieLS1]

I've had 8mm 125gr bullets completely exit and their SD is not good. I've had 30cal 165 bullets not exit on a similar size deer and the SD of that bullet is much better.

To me all that matters is shot placement and the ability of a bullet to expand and penetrate deep enough to hit the vitals. BC matters some to me but SD is an after thought.

 

[MCMXI]

Sectional density along with its companion weight retention is the #1 determinant of how deeply a bullet of any given type penetrates. That matters to the hunter, full stop. OP's post is absurd nonsense.

I knew I'd get this sort of response from those that love to throw SD numbers around.

And if your hunting bullets are tumbling, you've got a BIG problem.

You don't think that bullets can tumble once they enter an animal? You don't think SD changes once the bullet loses mass, expands or tumbles? This is what's absurd!

 

[MCMXI]

Does bullet weight matter at all? Do you believe that in general, heavier bullets penetrate more deeply than lighter ones? If you answer yes to either of those questions, SD matters.

You have to separate out the independent variables. Mass and velocity are independent variables.

 

[MCMXI]

I've had 8mm 125gr bullets completely exit and their SD is not good. I've had 30cal 165 bullets not exit on a similar size deer and the SD of that bullet is much better.

Pure heresy!

 

[Steve S.]

All of us that shoot, hunt and read a lot have a good idea of what caliber/ bullet weight is most efficient for most jobs. SD has never meant very much to me but I would not say that it is not important - I just see SD in a generalized application along with specific game, distance, velocity, shooting ability, etc. I do agree that marksmanship (bullet placement skill) is the most important factor of hunting, of accuracy - the sole purpose of marksmanship.

 

[Llama Bob]

You don't think that bullets can tumble once they enter an animal? You don't think SD changes once the bullet loses mass, expands or tumbles? This is what's absurd!

If your bullet is tumbling, it's time for a better bullet. You know, one of those high-SD weight retaining ones that you think don't matter

Seriously, it's clear you have ZERO grasp of terminal ballistics.

 

[MCMXI]

Seriously, it's clear you have ZERO grasp of terminal ballistics.

You're entitled to your opinion but it's interesting that test results exist that show SD is not a good indicator of bullet performance when comparing bullets of the same or similar weight, and yet there's no data to show that it is. I'm talking about bullets that have some form of controlled expansion. If you're going to claim that a .264 cal bullet with an SD of .293 is going to perform better than a .30 cal bullet with an SD of .264 simply based on SD then I don't agree with that.

 

[Llama Bob]

You're entitled to your opinion

Indeed I am, and that opinion of your claims is that you know less than nothing.

Then there are the laws of physics, which are more relevant to this discussion. And those have nothing to do with my opinion, being physical laws.

 

[MCMXI]

Indeed I am, and that opinion of your claims is that you know less than nothing.

Then there are the laws of physics, which are more relevant to this discussion. And those have nothing to do with my opinion, being physical laws.

It's hilarious how you desperately cling to the tenets that you dare not question. Once again, why not provide some actual data which I'm sure is founded firmly in the laws of physics that you mentioned.

 

[jmr40]

Elmer Keith and Jack O'Connor could never agree on these sort of things either. Its OK with me if we see things differently. But Jack killed everything with his 270 that Elmer killed with his 338.

 

[MCMXI]

Elmer Keith and Jack O'Connor could never agree on these sort of things either. Its OK with me if we see things differently. But Jack killed everything with his 270 that Elmer killed with his 338.

If enough's enough then not much else matters right? This forum is about presenting an opinion or idea, ideally with some data, and then allowing others to make up their own mind and possibly do their own research. The link that @Chuck R. posted and that I reposted below might be helpful. It's an article written by another individual that "knows less than nothing".

https://gundigest.com/gear-ammo/amm...od-measure-of-a-bullets-penetration-potential

 

[Llama Bob]

That article is as ignorant as you are, but at least he got paid for his ignorance - something to consider for the future.

The reality is that SD is useful not only for solids, but also for softs. Yes, you have to adjust for weight retention, and for softs that expand insufficiently or excessively. Everyone who's spent a minute thinking about this gets that. Then we get back to the physics, wherein the heavier bullets penetrate more. And once again, SD gets the job done and here we are

 

[WestKentucky]

BC and SD are only useful for long range. People who hunt at long range are typically looking to make a hit before worrying about the after effects of making the hit. BC is a means of measuring wind resistance which slows down the bullet as it travels. SD only matters here when looking at long bullets of multiple piece construction, OR if there is a large void (hollowpoint). What all of this means in reality is that the more aerodynamic a bullet is the further it carries its energy because it’s not being bled off by wind resistance as quickly. A heavy bullet has more mass and therefore more energy for the same velocity, so all of that put together is that a heavier and more aerodynamic bullet is favorable at longer range.... assuming the barrels twist rate approves of such bullet weight and velocity... and people have understood that for years.

 

[earplug]

Years ago I read a article on testing various bullets for brush hunting. The test results favored the higher SD bullets for being able to hit a obstruction and remain within the kill zone.

 

[Obturation]

I may be a bit off but I'll share my thoughts on SD, if anyone disagrees I'm going to my safe place with my blankie...
A sewer cap is what, 2 feet across? And weighs , let's just say 50#. It has a lower SD than a 1/4" rod (yeah, it would be long) of the same weight & material. Get hit with the sewer cap at going 100 fps you'll be knocked over but unpenetrated, get hit at the same speed with the unbelievably long piece of 1/4" rod, you will be run through easily. This is of course not realistic but if you exaggerate a concept in both directions it magnifies the results. So I do believe SD is important beyond avoiding being blown more easily off target by the wind. If I want penetration, I want heavy.
I could be totally off here but this is my understanding.