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

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.

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.

Read about the 6.5 MM bullet.

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

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

Haxby said:

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

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

Click to expand...

The experts at Sierra need to review their dimensions

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.

Mxracer239y -
You can look up the formula anywhere.

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

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.

"... 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.

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.

Click to expand...

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!

Click to expand...

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

Haxby said:

Mxracer239y -
You can look up the formula anywhere.

Click to expand...

: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.'

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.

I don't think sectional density is the answer.

Click to expand...

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

"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.

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.

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.

rcmodel said:

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

Click to expand...

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

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

Click to expand...

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.

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.

Click to expand...

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.