I have heard the term section density frequently, but do no know what it means or how it applies to effectiveness on game animals. Can someone explain?

Sectional Density is the mathematical calculation of the weight in pounds divided by the diameter of the bullet squared in inches.

It is relevant as an indicator of penetration. The higher the SD number the greater the penetration, given all other variables being equal (velocity, target media, bullet design and construction etc.)

So, a 180gr .308 diameter bullet has a SD of:

To convert grains to lbs you divide the number of grains by 7000

180/7000=0.025714 this is the weight in pounds

.308*.308= .094864 this is the diameter squared

Now divide the weight in pounds by the diameter squared

0.025714/0.094864= 0.271 That's the SD of this bullet.

Hope this helps.

fritz454,
Thanks for the education!

SBT,
Sectional density is only relevant to an object at rest. It has little use as a means of determining how a bullet will perform in flight or in tissue. Even with an object at rest the sectional density value will change depending on which side you view the object from.

i disagree, as it has to do with the frontal area of the bullet. Unless your bullet is traveling sideways, its a constant. Until, of course, it hits the animal and the frontal area changes. Which is why i think the sectional density as a measure of penetration in anything but solids is really pointless. What it comes down to is bullet construction. If your bullet expands a lot, you lose SD. What really matters is the sectional density after it mushrooms, not the sectional density everyone refers to befoer the bullet hits the animal. Just my opinion.

SeanD,
You are right. This is exactly why we do not put much value to sectional density numbers. To arrive at the SD the weight and the square area of one of the sides or ends of the object is needed for the calculation. SD therefore describes an object at rest.

The moment motion is brought into the calculation it becomes weight, frontal area and speed. This is something entirely different and it is a lot more reliable to use momentum as the determining factor in estimating penetration. Even then one is tripped up by the expansion characteristics of the bullet and what kind of nose shape it assumes after impact and how much weight it loses.

Often one sees that the ideal bullet will have this or that SD and it is used as a general rule of thumb. Two bullets of the same SD but differing construction will then assume wildly different SD values after impact. Similarly two bullets of the same construction and SD but at different speeds will penetrate to different depths if they are solids and do not deform. Makes a bit of a nonsense to even try and use SD as a value to estimate anything.

Gina,
Now, now, the sectional density is not totally useless! It made for a good parlor game once upon a time, called the Bwana Saeed Index, or BS Index, another one of those pseudo science lethality indices. It also makes for fun and games in the Penetration Index of Art Alphin.
BTW Chris Becker stole his index from the BS Index of yours truly.

The sectional density is actually an arbitrarily defined convention, a unitless number that has no physical reality itself. After all, bullets are not square in cross section. However, sectional density is directly proportional to a physical reality that involves the units of pounds per square inch, brought to bear by a cylindrical bullet sitting on it's base, at standard gravitational conditions. So, SD is not totally meaningless.

Gina, did you get my order squared away for those GSC FN and HV bullets? Best bullets in the universe, and that's no BS.

This will be the fourth order. Don't fail me now.

how do you figure its a unitless number? Wouldnt it be [F/L^2]? Since the area of anything is length squared.

Although i agree, it doesnt make any sense to use it for penetration calculations. The force is acting toward the ground, not the animal. An then you are missing a velocity somewhere in there. And then again, the frontal area is important, but so is the area on the sides of the bullet affecting the friction force in the opposite direction of the bullet. And then if the bullet mushrooms, everything is screwed up and all bets are off.

Sean,
Think about it. By convention or accepted definition of the term, when you calculate a sectional density, you are dividing the bullet weight by the bullet diameter squared. This is an imaginary surface area of a square bullet that you are dividing the weight of the bullet by. It is not a reality. It is a contrived expedient to make it simple to calculate without getting into:

Area of a circle = "pie are squared"

The conventional sectional density is directly proportional to a reality, but it is not a reality with real units. It is a unitless ratio. That is all.

I will say no more about this. This is so obvious, and we have hashed it out here before, several coon's ages ago, it seems.

To see the difference sectional density makes take a ping ping ball and a golf ball (they are about the same size but very different in sectional density), and throw each one as far as you can. A baseball and tennis ball will also work.

SD is important, and as Gina points out the number is not as constant as people think. Dangerous game hunting pundits usually call for a SD>.300 with a solid for the big boys.

Soft points are going to loose SD as they expand, but there should be some value they don't go below. The Nosler Partition performs very well in this regard, as the rear half of the bullet is a "solid" in case the front half is lost along the way.

I find the .375"/300 grain Nosler Partition to be a particularily useful bullet.

jim dodd

Sean,
Forgive me if I offended. So, I will say one more thing. The sectional density is a ratio, just like the ballistic coefficient is a ratio. A pure number with no units when its calculation is arrived at.

Again, my apologies. Some of us old farts get a little cranky when going over something for the umpteenth time. Time to move on to other topics, eh?

You guys are comparing apples to Onions, cannot be done, nor can one compare Monolithics to regular bullets....

Sectional Density is probably the most important factor in choosing a Woodleigh, Sierra, Hornady, Speer, Nosler, Hornady etc. for shooting big dangerous game....

And I would still take a 300 Gr. GS over a lighter GS bullet, same for Barnes X's, for Buffalo and I have used them and Barnes enough or observed their use enough to believe this..

The advise of using a lighter Monolithic bullet these days is IMO a big mistake, and mostly sales hype by producers of such...

I do believe Monolithics penitrate better than conventional bullets for the mostpart, but making them lighter for more velocity is not the way to go on Buffalo, elephant...I use and love the GS Custom bullets, both HP and FN solids, but I still like that weight pushing behind my bullet and my test show the 300 gr. out penitrates the 250 in the same bullet, both Barnes and GS... My test show the 300 gr. Failsafe, pentitrates better than their 270 gr.

I place a lot on Sectional Density because it works if you know how to read it, from a practical field application...