Sectional density

I don't really know how to formulate this question. I guess the best question I can come up with is what does sectional density have to do with external ballistics? What does it have to do with penetration and energy transfer?

For instance. I shoot a 6.5mm 140gr. bullet out of a 260. I would have to refer to my notes as to what the velocities are. This bullet is being listed as a sectional density of .287. A 220gr 338 bullet is listed as .281 and a 30 cal 180gr. is listed as .271.

I know that bullet design has a lot to do with expansion, energy transfer, etc... But this year I was astonished at the damage my 260 did to the deer it shot. And compared that with damage I have seen done to deer with other calibers such as 25-06, 30-06, 270 300 Win Mag, etc...I was wondering how much of that had to do with sectional density.

You've asked one question, but seem more curious about another.

Sectional Density is actually one of the primary determinants of a realistic ballistic coefficient for a long range bullet design. That is how it relates to external ballistics.

What you really seem interested in is its effects on terminal ballistics. The short answer is that it partially determines penetration. SD gives you an idea of the force that a bullet exerts in PSI. It essentially describes the bullet's potential for preserving momentum when it hits something. Consider a small finishing nail and a 10 penny nail. Which one is harder to drive into a block of wood? The finishing nail focuses the force into a small area while the 10 penny spreads it out more. Bullets work the same way. Where this analogy and serious consideration of SD alone for penetration breaks down is when you start looking at expanding bullets. As a bullet expands, its cross sectional area increases and its sectional density decreases. If the bullet expands rapidly, its sectional density decreases rapidly and the result is decreased potential for penetration.

helotaxi, thanks for that explanation. I guess you are right, I was more interested in terminal performance than external ballistics. I have been looking at the sectional density on bullets. And some bullets have high sectional density and some have lower.

So what you're saying is that sectional density in terms of penetration would really only matter if you were comparing FMJ bullets. Because once the bullet starts to expand the sectional density goes out the window... or at least changes rapidly. If I had a bullet with a lower sectional density but with low expansion characteristics it would penetrate deeper than a bullet with higher sectional density but extreme expansion characteristics.

Again, thanks for the explanation. It's what I had expected but I wanted to confirm.

Quote:

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Originally Posted by lomfs24

helotaxi, thanks for that explanation. I guess you are right, I was more interested in terminal performance than external ballistics. I have been looking at the sectional density on bullets. And some bullets have high sectional density and some have lower.

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

Good explanation there.