# RCmodel, Walkalong, or another expert please explain S.D.

gamestalker

December 14, 2011, 06:16 PM

I thought it had something to do with the length of the bearing surface and rate of twist but have no real idea how it computes.

Having reloaded for over 3 decades one would think I have a complete understanding, I don't and need some information about sectional density please. I have a bunch of reloading manuals, but I have never been able to find out what S.D. is and how, or if, it impacts bullet performance. Does it have an effect on a hunting round regarding bullet integrity such as penetration and, or, expansion.

I haven't had any problems all these years building loads that produce tight groups, but S.D. is something I've never acknowledged or researched as to how it effects reloading options.

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1911Tuner

December 14, 2011, 06:30 PM

I'll take a shot at it.

Sectional density is the ratio of a bullet's mass to its cross-section.

All else assumed to be equal, a 150-grain 7mm bullet has greater sectional density than a 150-grain .30 caliber bullet and will have a slightly flatter trajectory at a given muzzle velocity...and it will penetrate a little deeper in a given medium.

Likewise, a 180 grain .30 caliber bullet has a greater sectional density than a 165-grain .30-caliber bullet.

The actual difference in performance between the two bullets will be small.

Haxby

December 14, 2011, 06:30 PM

I'm no expert, but the experts at Sierra say:

Sectional Density: A bullets weight, in pounds, divided by its diameter in inches squared. High sectional density is essential to producing a good ballistic coefficient and deep penetration.

Red Cent

December 14, 2011, 07:06 PM

Read about the 6.5 MM bullet. :)

Walkalong

December 14, 2011, 07:06 PM

Sectional density is the ratio of a bullet's mass to its cross-section.Yep.

The longer a similar bullet is in the same caliber, the higher the sectional density, and the better the penetration potential, assuming similar construction and expansion.

Red Cent

December 14, 2011, 07:12 PM

http://www.chuckhawks.com/rifle_SD_list.htm

dmazur

December 14, 2011, 07:45 PM

Here's an article, and one of the definitions for BC is "sectional density divided by form factor". This will probably just contribute to the general state of confusion ( :) ), but I think it is interesting -

http://www.exteriorballistics.com/ebexplained/5th/221.cfm

rcmodel

December 14, 2011, 08:26 PM

Explaned another way with a question.

Say you have a 250 grain 50 cal round lead ball.

And a 250 grain 28" long, 1/4" dia arrow.

If both hit a target going 350 FPS, which one would you expect to penetrate fruther?

The arrow will shoot through a bucket of sand because it has a very high SD.

The larger dia round ball, not so much.

rc

Mxracer239y

December 15, 2011, 08:31 AM

I'm no expert, but the experts at Sierra say:

...divided by its diameter in inches squared...

The experts at Sierra need to review their dimensions :neener:

dmazur

December 15, 2011, 02:35 PM

Here's some more on sectional density -

http://www.gsgroup.co.za/sdplywood.html

An excerpt, after all the charts, says

Based on the last table, a direct comparison of penetration and sectional density in the chart above, shows that any link between the two is imaginary, as are comparisons of penetration to speed or momentum or energy. Terminal performance is a highly complex subject and an interaction of a multitude of factors. The closest one could probably come to a single factor for gauging terminal performance, is Momentum/Cross Sectional Area (Mo/XSA) and then only if the numbers are tempered with bullet shape, bullet construction and the effect of speed induced stagnation pressure.

The bottom line is that, choosing between two bullets based purely on sectional density, is as foolish as choosing a bullet based on the colour of the packaging.

Haxby

December 15, 2011, 03:02 PM

Mxracer239y -

You can look up the formula anywhere.

sugarmaker

December 15, 2011, 03:19 PM

SD = (weight in grains) / (7000*(d)^2)

d=diameter in inches

ex: 400 grain .50 cal

SD = 400/(7,000 * (.50)^2

=400/(7,000 * .25)

=400/1750

=.229

gamestalker

December 15, 2011, 03:53 PM

I forgot that my #10 Speer manual has just about every deffinition in the back pages of the book, but I'm still very grateful for the comparisons and explanations provided by you guys, as it has really helped me in understanding how SD effectively fits into my reloading and shooting needs.

Good stuff guys, and thanks again!

Merry Christmas,

Gamestalker

Jon_Snow

December 15, 2011, 04:03 PM

Just keep in mind that ballistic and terminal performance are going to be much more heavily influenced by other factors like bullet construction and shape.

ranger335v

December 15, 2011, 08:27 PM

"... the experts at Sierra say: Sectional Density: A bullets weight, in pounds, divided by its diameter in inches squared. High sectional density is essential to producing a good ballistic coefficient and deep penetration."

Well, they are experts but that's misleading, at best. High sectional density, of itself, means virtually nothing. A 180 gr. .30 cal bullet has a SD determined by those numbers, and it will be the same if it's loaded forward or backward but the BC and penatation at 200 yards will be much different.

All bullets of the same weight and diameter have indentical SD. The bullet's terminal effect is MUCH MORE dependant on construction than anything else! A fragile Nosler BT 150 gr. .30 cal has exactly the same SD as a 150 gr. .30 cal Barnes but the penatration will be much different. I've ignored SD as a meaningful factor in the choice of hunting bullets for about 40 years now, all I care about is the BC and construction for what I want to hit; if I want deeper penatration I'll use a bonded core, a partition or a monolithic type bullet.

1911Tuner

December 16, 2011, 03:47 AM

A 180 gr. .30 cal bullet has a SD determined by those numbers, and it will be the same if it's loaded forward or backward but the BC and penatation at 200 yards will be much different.

I think it's safe to assume that they mean bullets loaded nose forward.

All bullets of the same weight and diameter have indentical SD. The bullet's terminal effect is MUCH MORE dependant on construction than anything else!

It's also probably safe to assume that the comparison is made with the standard "All else assumed to be equal" disclaimer.

Mxracer239y

December 16, 2011, 07:21 AM

Mxracer239y -

You can look up the formula anywhere.

:banghead:

You can. What you will find is that you divide weight by diameter squared (in dimensions of length squared). You do not divide by diameter, as quoted (which would have dimensions of length).

The quote mentioned diameter having dimensions of length squared. It is a confusion between the 'diameter in inches squared' and 'diameter squared.'

dmazur

December 16, 2011, 07:49 AM

I found the "lead" article, before the plywood tests on penetration -

http://www.gsgroup.co.za/articlesd.html

This one is titled "Sectional Density - A Practical Joke?" and I think it is worth reading.

The conclusion seems to be that yes, one can calculate sectional density, but it doesn't really appear to have any practical application. Nevertheless, the term is still tossed around as if it did.

There are better measures of bullet performance, but shooters seem to want a single, all-inclusive term.

I don't think sectional density is the answer.

1911Tuner

December 16, 2011, 08:28 AM

I don't think sectional density is the answer.

It's not. It's basically just a piece of information that means little by itself.

ranger335v

December 16, 2011, 12:32 PM

"A 180 gr. .30 cal bullet has a SD determined by those numbers, and it will be the same if it's loaded forward or backward but the BC and penatation at 200 yards will be much different."

"I think it's safe to assume that they mean bullets loaded nose forward."

So we would assume but what "they mean" wasn't the point; loaded either direction doesn't change the SD and that IS the point!

My example simply proves that bullet weight, per se, provides no automatic benefit to the BC. (Boat tails generally add to BC but not so much on backward spitzers, which would effectively make them boat tail wad-cutters!)

"All bullets of the same weight and diameter have indentical SD. The bullet's terminal effect is MUCH MORE dependant on construction than anything else!"

"It's also probably safe to assume that the comparison is made with the standard "All else assumed to be equal" disclaimer."

Yeah. But it's also safe to assume that all things are rarely "equal." I'm talking real world bullets here, not isolated ideas in some anal theory.

Haxby

December 16, 2011, 01:40 PM

Chuck Hawks - To calculate a bullet's sectional density divide the bullet's weight (in pounds) by its diameter (in inches), squared.

Merriam - Webster Dictionary - Definition of SECTIONAL DENSITY

: the ratio of the weight of a projectile to the square of its diameter

(Lilja) riflebarrels.com - sectional density is defined: SD=w/d^2

Hornady - Sectional density (a bullet’s weight in pounds divided by its diameter squared)

Nosler - The ratio of a bullet's weight, in pounds, to the square of its diameter, in inches.

Speer - A bullet's weight in pounds divided by the square of the diameter in inches.

There is no mention of area.

denton

December 17, 2011, 12:33 AM

There is no mention of area because the area of a circle is directly proportional to its diameter squared. Since SD is just a comparative number, there is no point in also applying the pi/4 terms. Remember, this was invented back when there were no four function calculators.

SD does influence BC. Long skinny bullets have higher BC and higher SD.

The other use for SD is to answer the question, if I like an X grain bullet in my Y rifle, what weight bullet gives the same proportions in the Z rifle? So, IIRC, if you scale a 180 grain 30 cal bullet down to 7mm, you get something like 162 grains.

ArchAngelCD

December 17, 2011, 12:38 AM

Explaned another way with a question.

Say you have a 250 grain 50 cal round lead ball.

And a 250 grain 28" long, 1/4" dia arrow.

If both hit a target going 350 FPS, which one would you expect to penetrate fruther?

The arrow will shoot through a bucket of sand because it has a very high SD.

The larger dia round ball, not so much.

rc

The picture drawn here with words makes SD very clear IMO.

dmazur

December 17, 2011, 01:33 AM

The picture drawn here with words makes SD very clear IMO.

Yes, it is a clear picture. I'm not sure it is at all related to bullet performance, however.

I'm beginning to believe that SD is only part of the story, and discussing it as an isolated entity is misleading at best.

1911Tuner

December 17, 2011, 04:41 AM

Yeah. But it's also safe to assume that all things are rarely "equal." I'm talking real world bullets here, not isolated ideas in some anal theory.

We understand that. The OP's request was: "Please define Sectional Density." So defined, all else must be assumed to be theoretically equal in order to satisfactorily explain it. That being: Assuming equal construction...range...initial velocity...impact velocity...atmospheric conditions...and target medium, the bullet with higher sectional density will shoot a little flatter and penetrate a little deeper.

"Anal" theories over bullet performance in the real world pretty much fall under another topic. We can add that actual bullet performance depends more on construction...range...initial velocity...impact velocity...atmospheric conditions..and target medium without going into firing into ballistic gel vs a living animal or silliness such as loading the bullet backward which nobody in the real world is going to do.

:)

Walkalong

December 17, 2011, 08:58 AM

The longer a similar bullet is in the same caliber, the higher the sectional density, and the better the penetration potential, assuming similar construction and expansion.

The 7MM Mauser & the 6.5X55 Swede both have a reputation for killing power due to their long heavy bullets with high sectional densities.

Whether that is why they do a good job can be debated for all time, but I have no doubt that the high SD's help.

But then again, I am a believer in heavier, well constructed bullets that pass through the animal doing a lot of damage all the way through, vs bullets that "expend all their energy in the animal" without passing through, which is a faulty premise, IMHO.

Yes, both can kill game, never say they won't, just expressing my preference, and which I believe is better.

To me, sectional density is something to look at when choosing bullets. Not everything by a long way, but something to consider when choosing between good bullets.

helotaxi

December 17, 2011, 10:26 AM

I found the "lead" article, before the plywood tests on penetration -

http://www.gsgroup.co.za/articlesd.html

This one is titled "Sectional Density - A Practical Joke?" and I think it is worth reading.

The conclusion seems to be that yes, one can calculate sectional density, but it doesn't really appear to have any practical application. Nevertheless, the term is still tossed around as if it did.

There are better measures of bullet performance, but shooters seem to want a single, all-inclusive term.

I don't think sectional density is the answer.

That article is a fantastic example of using half of the facts and an air of expertise to provide essentially false or misleading information. Not sure what the author's agenda was there but they certainly had one, and it wasn't providing knowledge based on objective science.

To paraphrase: Since a varmint bullet and a bonded hunting bullet have the same weight and sectional density, they should penetrate the same. Since they don't sectional density doesn't mean anything.

Odd. I thought that only proved what every hunter knows "use the right bullet for the job". Bullet construction matters. A lot. Which takes us to...

Again paraphrasing: Since the bullet's sectional density decreases as it mushrooms and it's more effective (without really defining what effect it being sought) the more it mushrooms, sectional density doesn't mean anything.

This is the bullet construction argument all over again. Sectional density determines penetration for a given velocity. Period. If the bullet deforms rapidly, the sectional density decreases exponentially. In flight, sectional density is constant. Once it hits a target it changes as the bullet deforms. It will also only change for the worse, either the bullet sheds weight (linear decrease if it doesn't expand at the same time) or the cross sectional area increases (causing an exponential decrease assuming that it doesn't shed any weight). These changes in sectional density are what determine the penetration differences between two bullets of different construction with the same static SD.

The closest one could probably come to a single factor for gauging terminal performance, is Momentum/Cross Sectional Area (Mo/XSA)...

Since momentum is mass x velocity...holy cow! Sectional Density x velocity is the single factor for gauging terminal performance!

ranger335v

December 17, 2011, 10:37 AM

"Assuming equal construction...range...initial velocity...impact velocity...atmospheric conditions...and target medium, the bullet with higher sectional density will shoot a little flatter and penetrate a little deeper."

Yes, it would. But it's due to a higher mass/intertia, not the SD number per se.

Again, any two bullets of the same diameter and weight have identical SDs but their construction easily varies a LOT and that is what would limit penatration. In fact, a .30 cal 150 gr. FMJ will always out penatrate an equal weight 170 gr. round nose made for the .30-30 even tho the 170 will have a higher SD. Therefore, a hunter had best use the bullet constuction to guide his selection, not the SD.

Sectional Density had a meaning when the measure was derived from lead bullets up to early jacketed bullets around maybe the late 1930s. All early jacketed bullets were of virtually identical construction but that hasn't been true for several decades now. Meaning, IMHO, differences in construction have long made SD irrelivant as a valid bullet selection criteria.

No matter, it's all fun to play with.

Jim Watson

December 17, 2011, 10:46 AM

Sectional density used to mean more back when bullets were all of the same cup and core construction. Higher SD usually meant better penetration.

That all started to change when Nosler brought out the Partition which was one of the first to divorce bullet performance from sheer mass. And it has been getting more pronounced as the patent bullets proliferated. You can get as good or better results with lighter bullets of good construction than an ordinary softpoint. Not that the game has been getting tougher. A good softpoint will still kill, but we now expect greater accuracy and flatter trajectory.

1911Tuner

December 17, 2011, 10:48 AM

Yes, it would. But it's due to a higher mass/intertia, not the SD number per se.

Assuming that the higher SD comes with a longer, heavier bullet in the same caliber, and not in a smaller caliber with a longer bullet of the same mass.

i.e. The 150-grain 7mm vs 150-grain .30 caliber that I referenced.

At the same muzzle velocity they'd have equal momentum. By the time they made it a hundred yards, the 7mm would be carrying a bit more due to its superior ballistic coefficient and smaller frontal area.

Again...all he asked for is a definition of sectional density, and how it affected the bullet's in-flight and terminal performance. In order to answer the question and allow him to understand it better...we assume that all else is equal.

Jim Watson

December 17, 2011, 11:05 AM

That assumes that "all else" includes form factor.

A 7mm roundnose will have higher SD than a .30 spitzer of the same weight, but lower BC and poorer external ballistics.

But then one of the gunzine writers showed by actual shooting that a spitzer had no significant advantage in trajectory over the first two hundred yards, versus a roundnose.

1911Tuner

December 17, 2011, 11:26 AM

Jim...True that. The SD number is just a little pertinent information included with the bullet profile in a loading manual...and not much else...in case anybody should want to know.

As far as using it to calculate bullet performance, it can be plugged in...but after all that cipherin' and head-scratchin'...there just ain't enough difference between a 165 grain .30 caliber bullet at 1500 fps and a 150-grain .30 caliber bullet to make a practical difference...

Wait for it...

Here it comes...

All else assumed to be equal.

helotaxi

December 18, 2011, 09:39 AM

"Assuming equal construction...range...initial velocity...impact velocity...atmospheric conditions...and target medium, the bullet with higher sectional density will shoot a little flatter and penetrate a little deeper."

Yes, it would. But it's due to a higher mass/intertia, not the SD number per se.

Mass and inertia directly play no part in exterior ballistics. SD does because it, with form factor, dictate BC. It's a common misconception that heavier bullet resist the wind better. Assuming the same general form factor and caliber, that is true, but it is because the increase in weight increases the sectional density and thus the BC.

Again, any two bullets of the same diameter and weight have identical SDs but their construction easily varies a LOT and that is what would limit penatration.Because their differences in expansion characteristics change the SD. In fact, a .30 cal 150 gr. FMJ will always out penatrate an equal weight 170 gr. round nose made for the .30-30 even tho the 170 will have a higher SD. Therefore, a hunter had best use the bullet constuction to guide his selection, not the SD.To analyze the effect that a variable has on an outcome, you can't merely ignore all the other variables out there and note the change in the variable of interest. The other variables have to be controlled for. If you want to examine the effects of SD on penetration depth, you have to control for form factor, impact velocity and expansion characteristics. It's a simple matter to find exceptions or outliers to make a point, but since there is no control in place the point isn't valid. A hunter should choose his bullet based on a ton of different variables and if he requires deep penetration from the bullet, SD had better be part one of them. Look at bullets of similar construction but different SDs to be able to draw a valid comparison.

Let's look at the 7mm 154gn Hornady IB (SD .273) fired from a 7mm-08 and the .308 cal 155gn IB (SD .233) fired from a .308 Win. Same bullet weight (essentially) same bullet construction, same form factor, same velocity from the muzzle (though the 7mm will start to pull away as the range increases thanks to the superior BC provided its higher SD). Which is the better choice for medium to large game where the goal is to make sure the bullet gets to the vitals from a wide range of possible shot angles?

Sectional Density had a meaning when the measure was derived from lead bullets up to early jacketed bullets around maybe the late 1930s. All early jacketed bullets were of virtually identical construction but that hasn't been true for several decades now. Meaning, IMHO, differences in construction have long made SD irrelivant as a valid bullet selection criteria

Only because people don't understand what it means and how it allows for a comparison within like constructed bullets across different calibers.

Jim Watson

December 18, 2011, 10:25 AM

In fact, a .30 cal 150 gr. FMJ will always out penatrate an equal weight 170 gr. round nose made for the .30-30

That theory got some intrepid hunters eaten and stepped on in the early days before consumer chronographs and gelatin blocks, when power was judged by how many boards a bullet would penetrate.

A hard spitzer - I don't know many .30 150 gr FMJ RN to keep everything "equal" - tends to yaw, deflect, "tumble," and range around unpredictably upon impact. It might run up more total inches of penetration than a roundnose softpoint but they might not be where you want them.

All else seldom is equal.

helotaxi

December 18, 2011, 10:43 AM

And then the "backup" bullet when the expanding bullet didn't get the job done was usually a solid. Sometimes you have to give up wound channel and energy delivery to make sure the bullet gets to the vitals. A .45 cal hole in the heart beats the bullet not getting there every time. The round and flat nosed bullets were found to track straighter through flesh.

Bullet expansion, though, is still a matter of managing the change in SD to meet a certain goal, be that maximum energy delivery requiring "total" expansion and a nearly infinite increase in cross sectional area and decrease in weight running SD toward zero, or the deepest penetration possible requiring minimal expansion and little or no increase in area or loss of mass, or most likely somewhere in between. SD will only decrease as a bullet expands or fragments, so an expanding bullet needs to have more than enough to start with to meet the penetration goal while allowing for expansion.

OkieGentleman

December 18, 2011, 11:25 AM

Congratulations!! This hair has been split about 20 times and not one angry word was typed. I love reading these discussions, especially when everyone stays focused and there are no flames generated. :D Now if we could just get the politicians to do the same thing!:banghead:

1911Tuner

December 18, 2011, 11:48 AM

Hair splittin' is almost inevitable. It's come to be expected on most technical threads.

Whenever a poster doesn't have the basic understanding of a term, the first task is to remove anything from the equation that doesn't directly address it. If it gets cluttered up in semantics, it can be confusing. In order to do that, we eliminate everything except the base definition of sectional density...which is essentially a simple ratio of mass to length...take a given pair of SD numbers, and assume all else to be equal, even if only hypothetically...and compare the two.

Once he understands the basics, he can then apply it in the practical sense and factor in the variables of range...initial velocity and impact velocity...target medium and target angle, etc. Even then, there are variables that can't be controlled or factored in with any degree of accuracy. i.e. Whether the bullet hits a massive or a light bone, or whether the bullet slips between two ribs or hits one squarely, etc. Everything means something.

Walkalong

December 18, 2011, 02:59 PM

if we could just get the politicians to do the same thingHeck, if we just had politicians as smart as some of the folks around here. :)

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