# Ballistics: Sectional Density



## Bob Wright (May 10, 2006)

Bear in mind, the following is only my understanding of the subject:

Sectional density is the weight of a bullet compared to its cross sectional area. The cross sectional area of a .30 caliber bullet is the area of a circle having a diameter of .30". The area of .30 caliber bullets remains the same for all bullets of .30" diameter. Weights vary, from, say 150 grains to 220 grains. The heavier the weight, the greater the sectional density, simply W/A. The greater the sectional density, the better able a projectile to penetrate, neglecting bullet expansion.

Consider this: Noted archer Fred Bear used to demonstrate using a bucket of sand. He'd fire a .30-06 rifle into the bucket, catching the bullet in the bucket about midway or so. He then shot an arrow into the bucket, penetrating both sides. The rifle bullet had much greater initial velocity, but the arrow had greater sectional density.

Same applies to the armor piercing round now used for the 120mm gun on the Abrams tank. The old 90mm Hypershot tank round used on the M-47 Patton tank was hardened steel with pre-engraved driving/rotating bands fired at around 4000 fps. The new sabot rounds have a dart that is relative small in diameter but long, and very heavy, at about 5000 fps velocity. It is the higher sectional density of the sabot round that gives it its penetration and destructiveness.

Try shooting fish in water. The bullet won't go much over a foot below the surface, then looses all of its velocity, falling to the bottom only because of gravity. Yet, archers regularly take fish with archery tackle. If you do try this, wear a waterproof jacket and hat. All that water thrown up by the bullet comes right back down.
Bob Wright


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## Orion6 (Jul 29, 2006)

Hi Bob,

Thanks for your explanation which is very clear and has helped me to properly understand just what you mean by sectional density.

How would this apply to the tubular bullet scenario? (Sorry, I don't know which thread is more relevant now!)

A tubular bullet of any given length is going to have a higher velocity, lower weight, and smaller cross-sectional area than its conventional counterpart. I guess only empirical testing would tell us just how it all pans out...

Bill


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## Orion6 (Jul 29, 2006)

By the way, your comment about shooting fish is very relevant. A guy at the range who claimed to have actually experimented with tubular projectiles related how his test tank suffered dings in the bottom where conventional projectiles lost all velocity and sank before the bottom of the tank...


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## Bob Wright (May 10, 2006)

Purely conjecture, but any advantage of tubular bullets is going to be close range, at handgun ranges. And most ballistic studies concern rifle velocities and ranges.

True, being lighter, a tubular bullet could be launched at higher velocity, within the same pressure limits, than a heavier bullet. But, lighter bullets shed their velocity faster than heavier bullets. At close range this might be inconsequential.

Its hard to imagine any advantage such a bullet might offer over conventional. I did note the one reference I found were with .30 caliber bullets "from the 1890s" that had been loaded by Lake City Arsenal in 1958 in .30 caliber (.30-06) cases. Obviously for military testing, though I'm not aware of any such test. That, incidentally, would have been about the same time of my military service.

Bob Wright


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