Trying to explain sectional density to the dummies at work

[stringnut]

My wife just traded her 308 for a 6.5 CM. The guys I work with just cannot understand why she give up "all that power " of the 308. I understand that sectional density makes a big difference in penetration. I also know that it is the ratio of the diameter to the weight of the projectile. I cannot get it across to them that a 140 6.5 will penetrate better than 150 308 at the same speed and same construction. Any good way to explain this to people who refuse to think or listen. Their argument is that the bullet is heavier ,so, has to be better. For all practical purposes it makes little or no difference. Nothing up to and including elk are going to know the difference at the distance she is likely to shoot. Just trying to convince the lads she hasn't handicapped herself.

 

[swampcrawler]

Grab a bowling ball and a 3 ft piece of 1/2 rebar or round stock. Drop both onto grass/dirt from a few feet. Measure penetration.

Obviously you probably can’t actually do this at work but maybe try explaining with some similar analogy that the average guy can envision?

 

[Demi-human]

When you figure this out, can you tell me how to explain "Ballistic coefficients", also to dyed in the wool, "Elevation Williams"?
Great guys, I am most sure of, but you may be sooner able to change the wind. (In fact, I'd rather shoot with no one else, for I am quite Hillbilly as well!)

 

[Demi-human]

I think a fine Mount, taken with the afore mentioned rifle, would be very persuasive...

 

[jmr40]

You have my permission to print my photo. It says it all. The bullet on the left is a 143 gr 6.5. The next bullet is a 150 gr 308. It is bullet LENGTH that determines penetration, not diameter or weight. The 150gr fired from my 308 achieves 2850 fps, the 143 from the 6.5 CM 2650 fps. But at only150 yards the 6.5 is moving faster and with more energy. The gap widens as range increases. The 6.5 bullet not only penetrates deeper, it penetrates MUCH deeper at any range. Also I'm burning 5 gr less powder pushing a 7 gr bullet 200 fps slower. The 6.5 has significantly less recoil.

The next bullet in the lineup is a 178 gr .308. In my 308 it leaves the muzzle at the same speed (2650 fps) and since it is close to the same length penetration is very similar. It also has more energy at close to moderate ranges. Note the key phrase here, You need a 178 gr bullet in a 308 just to MATCH what the 143 does in a 6.5. But now I'm burning 3 gr more powder to push a 35 gr bullet to the same speed. Recoil is now significantly more, and the 6.5 is a lot flatter shooting.

The bullet on the far right is a 200 gr bullet. IF you could get it moving fast enough you now have a bullet that exceeds what a 143 gr 6.5 bullet does. And fired from a 300 magnum it just barely beats 6.5 Creedmoor downrange. But you can't get a 200 gr moving fast enough in a 308 to take advantage of its capabilities.

 

[stringnut]

Jmr , your picture shows it all. I may take a couple of different bullets in to show them. Your tag line really explains their attitude. " most people don't really want the truth" explains it PERFECTLY!

 

[TN Outlaw]

There is a group of 2-3 guys at my work that its been one stupid thing after another that's came out of their mouth as it pertains to firearms and ballistics. The ones that know the least always seem to be the most vocal in my experience.

People have differing opinions and that makes for good conversation but trying to dispute these common facts just shows they have no buisness arguing. The main thing to remember is these people dont know enough to know that they don't know squat! They would actually have to put forth some time and research just to figure out how clueless they are so no real sense in trying to go too far out of your way explaining these things.

 

[someguy2800]

I personally would save my breath. People all have opinions and fantasies about what works best and I’m no exception. Some people think a 300 win mag is barely suitable for jack rabbits and others are convinced there 243 would stop a charging grizzly.

My opinion Is that once you get over a certain point sectional density is a measurement of how deep a hole you put in the dirt behind the deer. I’ve only ever recovered one bullet

 

[Varminterror]

How to explain sectional density and penetration to the simplest of layman:

Hit a board with a hammer. Now hit a NAIL into the board with a hammer. Which one penetrates better?

That’s really a comparison of psi, but the penetration comparison is immediately relatable for anyone.

Sectional density, for a more advance layman (oxymoron?), is a ratio of the weight divided by the cross-sectional area. It’s effectively a measure then, of how widely distributed the weight of the bullet is. The wider the weight is spread, the more surface area it’s force is distributed over, and now we’re back to PSI and the hammer and nail analogy above.

 

[Don Dayacetah]

I see one of three dynamics.
1. They're trolling you, just trying to get your goat.
2. They're actually that stupid.
3. They prefer the wider cross section of the larger bullet,
and penetration isn't as important as the larger caliber.

 

[HankC]

Those dummies on penetration may be thinking of wound channel and energy transfer, there is always a trade off, which is better depends on what you want to do and if the benefit worth the trouble.

 

[CraigC]

SD is the ratio of diameter to weight. A 150gr 6.5 penetrates better than a 150gr .308 because it's momentum is exerted across a smaller surface area and because the bullet encounters less friction once it reaches the target. Which is the easier nail to drive with a hammer, a 16 penny, or a finish nail?

Or just give up because some people just don't get it.

It is bullet LENGTH that determines penetration, not diameter or weight.

No it isn't. It's the weight to diameter ratio, which is what sectional density is. It's the bullet's momentum, coupled with several other factors like construction and nose shape, that determine penetration. Not length. High SD bullets 'usually' penetrate better because they have a high weight to diameter ratio. Not because they are long.

 

[Steve S.]

The deer that I have taken with my .308 have never stopped a round at any distance; all of the deer that I have taken with my 6.8 SPC have stopped all of the rounds (they tend to fragment in the chest cavity). Most of the .308 deer act like they are not even hit and run pretty well for 30 - 50 yards or so; the 6.8 deer always appear to be hit hard and either drop or walk a few steps and then drop. My field experience tells me that all of the incremental ballistics science that is discussed for all the various rounds is moot in the field but it is fun to read about.

 

[Varminterror]

I can appreciate some folks might consider this to be splitting hairs, but it does irk me when folks throw around terms which seem to add credence to their statements, but are using the terms incorrectly - let alone having them upside down.

“Ratio of diameter to weight” get thrown around a lot, but it’s not correct. “Ratio of diameter to weight would be D:W. Ratios are a relative count of two populations. If we say “ratio of men to women” and we have 2 men and 1 woman, it’s 2:1. So then as fraction/proportion/percentage we’d have a population of 2/3 men, 1/3 women, and 67% men, 33% women. So diameter to weight is simply D:W, mathematically it would show as D/(D+W) + W/(D+W) = 1. Similarly, ratio of weight to diameter isn’t correct either - again, it would end up as W, so the math would be the same 1 = D/(D+W) + W/(D+W). That’s not correct - because sectional density isn’t a ratio.

As familiar as it might sound for laymen to say “ratio of diameter to weight,” that’s not sectional density.

Sectional density is the quotient of bullet weight in pounds over the diameter in inches squared.

SD = Wt/D^2.

That’s not a ratio. It’s a quotient. And diameter isn’t in the numerator, it squared is the denominator.

 

[someguy2800]

Talk of sectional density and penetration also ignores bullet construction and velocity. A nosler partition and a nosler ballistic tip of the same sectional density are not going to penetrate the same.

 

[22250Rem]

I once heard sectional density explained using examples like an arrow from a modern compound bow on a broadside lung shot on a whitetail deer. The velocity is so low compared to a bullet that those who aren't familiar with such stuff would wonder why it would penetrate as much as it does. The answer is sectional density and it was many years ago I heard this explanation and it made great sense. Now if I could only recall the details of how it all applies in this situation.

 

[JohnKSa]

Any good way to explain this to people who refuse to think or listen.

Sadly, no.

That’s not a ratio. It’s a quotient.

I suppose this is why numbers that can be expressed as the quotient of two integers are called 'quotiental numbers' and numbers that can't be are called "irquotiental numbers".

 

[swampcrawler]

The deer that I have taken with my .308 have never stopped a round at any distance; all of the deer that I have taken with my 6.8 SPC have stopped all of the rounds (they tend to fragment in the chest cavity). Most of the .308 deer act like they are not even hit and run pretty well for 30 - 50 yards or so; the 6.8 deer always appear to be hit hard and either drop or walk a few steps and then drop. My field experience tells me that all of the incremental ballistics science that is discussed for all the various rounds is moot in the field but it is fun to read about.

Not to derail the thread, but the 20 or so deer I’ve taken with the 308 have all but two dropped as if struck by lightening(Though I also never recovered a bullet.) The two exceptions were that time I discovered behind the shoulder lung shots kinda suck.

Anecdotal evidence can be just as flimsy as ballistic data in the real world, as two people are just as likely as not to have totally different results under apparently similar circumstances.

P.O. Ackley swore the 220 swift would be his only rifle, could he only have one, after one shot dropping a bunch of wild donkeys with body shots. I dunno. Shooting is part science, part art, and part black magic.

 

[swampcrawler]

Sadly, no.I suppose this is why numbers that can be expressed as the quotient of two integers are called 'quotiental numbers' and numbers that can't be are called "irquotiental numbers".

I always thought the ones that couldn’t were just “ “unreasonable”.

 

[CraigC]

I can appreciate some folks might consider this to be splitting hairs, but it does irk me when folks throw around terms which seem to add credence to their statements, but are using the terms incorrectly - let alone having them upside down.

“Ratio of diameter to weight” get thrown around a lot, but it’s not correct. “Ratio of diameter to weight would be D:W. Ratios are a relative count of two populations. If we say “ratio of men to women” and we have 2 men and 1 woman, it’s 2:1. So then as fraction/proportion/percentage we’d have a population of 2/3 men, 1/3 women, and 67% men, 33% women. So diameter to weight is simply D:W, mathematically it would show as D/(D+W) + W/(D+W) = 1. Similarly, ratio of weight to diameter isn’t correct either - again, it would end up as W, so the math would be the same 1 = D/(D+W) + W/(D+W). That’s not correct - because sectional density isn’t a ratio.

As familiar as it might sound for laymen to say “ratio of diameter to weight,” that’s not sectional density.

Sectional density is the quotient of bullet weight in pounds over the diameter in inches squared.

SD = Wt/D^2.

That’s not a ratio. It’s a quotient. And diameter isn’t in the numerator, it squared is the denominator.

Let's maintain some context and perspective. This ain't a graduate level math class. We're talking about people who can't even grasp the concept. People the OP referred to as "dummies" and you're gonna throw out ten dollar words like quotient? Everyone knows what "ratio" means. If I had said quotient, how many would have to have that explained to them or would have to look it up? "Ratio" might not be technically correct in a mathematical environment but it's far more illustrative to the average "dummy" than quotient.

Most people are going to Google it and find Wikipedia where it's referred to as a "ratio". Sometimes splitting hairs serves a useful purpose. Sometimes it doesn't.

https://en.wikipedia.org/wiki/Sectional_density

 

[WestKentucky]

Probably also the guys who stand behind “loud pipes save lives” and “no replacement for displacement”. It’s hard enough for me to convince them my Toyota truck was built in Texas instead of “enemy territory”.

I feel your pain, but at the same time I don’t really care for the 6.5s. Not that I dislike them, I just don’t really see much benefit over something like a .277 or 7mm bore. For most hunting applications your looking at penetration, expansion, and energy transfer. If you overpenetrate you wasted energy. If you fragment badly (impact detonation) then you dump energy quickly and sometimes don’t get that energy where it needs to go. It’s all a trade off, but my experiences have been that when you go to one extreme you pay dearly with the others. 6.5 isn’t to that degree of an extreme, but I don’t see the trade off being a perfect solution either. Certainly not detrimental to a point of calling it a handicap, but it’s also not the mythical wonderbullet some folks make it out to be. My biggest concern is and always will be stopping the bullet where I want it to stop. Just under that skin on the far side. Anything more and I worry about collateral damage.

 

[CraigC]

For most hunting applications your looking at penetration, expansion, and energy transfer. If you overpenetrate you wasted energy.

Here's another myth.

Heavy bullets in the 6.5mm diameter are nothing new and yes, there is a lot there. The .270 is a deer cartridge, elk on the extreme end. The fast twist of the various 6.5's is what makes high SD, deep penetrating bullets usable. Do some reading about the seemingly mild 6.5x54MS.

Loud pipes do save lives and there is no replacement for displacement. Doesn't matter where it's assembled, a Toyota is still a Japanese truck. Is a Harley made in Brazil a Brazilian motorcycle or an American Harley?

 

[WestKentucky]

Here's another myth.

Heavy bullets in the 6.5mm diameter are nothing new and yes, there is a lot there. The .270 is a deer cartridge, elk on the extreme end. The fast twist of the various 6.5's is what makes high SD, deep penetrating bullets usable. Do some reading about the seemingly mild 6.5x54MS.

I must be missing something or misreading your post.

If your impact energy is 1000 (units don’t really matter) and that’s a mathematical calculation of of speed (assume 100) and mass (assume 10). Lets be pretty realistic and say that when we shoot an animal and poke a hole on the back side that we cut our velocity by about 2/3 and our bullet shed about 30% of its mass. So now we are at speed 34 and mass 7. We are left with 238 units of energy that were available to use, but were not deposited in the animal. Didnt we just waste 23.8% of our available energy? Again, my fear about this is that 238 units of energy might find their way to a neighbors house, another animal, or heaven forbid another human. The shockwave produced from an impact does mangle the body pretty good for a few inches in every direction but once a bullet starts mushrooming and slowing down that fades away to more of a tearing wound channel where your bullet literally rips a hole in the animal.

And yes, .270 is typically seen as a whitetail round, and with the 130gr bullets most .270 win guns like I would tend to agree that it probably isn’t optimal for elk although it has been used successfully for years. The 150 gr bonded bullets and copper bullets (Barnes) ratchet the gun up into elk territory nicely.

Beyond this point the discussion becomes a urination for distance competition about whether energy or some other phenomenon is responsible for abruptly anchoring dinner.

 

[CraigC]

You have to believe that energy is a meaningful number to go down that rabbit hole. I don't. All that matters is the depth, breadth and location of the wound channel. The quantity of energy consumed or "wasted" in the process is irrelevant.

 

[Demi-human]

I always thought the ones that couldn’t were just “ “unreasonable”.

I have a reasonable dislike for irrational numbers.