Ballistic Coefficient


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Nightcrawler
February 25, 2003, 11:00 PM
What exactly is ballistic coefficient? What is it a measure of? What does it mean?

As I understand it, a bullet with "better" ballistic coefficient will penetrate better, and is often achieved by having a projectile that's longer than it's bore diameter.

I've heard that adding fins to a projectile ruins ballistic coefficient.

I'm confused. Help!

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JohnKSa
February 25, 2003, 11:46 PM
Ballistic Coefficient is the measure of how aerodynamic a projectile is.

Here are some general rules of thumb for what makes ballistic coefficient better.

If you hold everything else equal.

A heavier projectile is more aerodynamic
A projectile with less frontal area is more aerodynamic
A projectile with a more tapered back end is more aerodynamic

Generally speaking, a more aerodynamic projectile will also penetrate better unless

1. It tumbles on impact.
2. It deforms on impact.

Fins create more drag, but can stabilize a projectile which might otherwise be unstable. An unstable (tumbling) projectile will not be aerodynamic at all.

P95Carry
February 25, 2003, 11:53 PM
If you want a bit more info still .. and a loada math!!! Try this page ....

Ballistic Coefficients (http://www.aeroballisticsonline.com/ballistics/bc1.html)

MCNETT
February 25, 2003, 11:57 PM
Ballistic Coefficient is the bullets ability to defeat wind drag, thus hold its velocity as it travels through the air.
Sectional density is what you are referring to in regards to penetration. Also, all bets are off on SD dictating better penetration when expanding bullets are used. Though it is a measuring stick in the industry.
-Mike

Nightcrawler
February 26, 2003, 12:33 AM
OH...it's aerodynamics. I understand that. I used to be a pilot, believe it or not.

Okay.

So...sectional density, then?

JohnKSa
February 26, 2003, 01:07 AM
Long thin heavy projectiles have high sectional density which means that they will tend to penetrate very well if they don't tumble or deform.

Short fat light projectiles have low sectional density.

Sectional density is one of the characteristics that figures into the determination of a ballistic coefficient (I'm not really saying that right, but I can't figure out a better way to explain it). The higher the sectional density, in general, the better the ballistic coefficient.

boing
February 26, 2003, 01:14 AM
And how is the sectional density of an object derived?

EDIT: from the above page, the answer is: Sectional density is equal to the bullet's weight in pounds divided by the square of the bullet's diameter; SD = w ÷ d².

Matthew Courtney
February 26, 2003, 03:45 AM
Sectional density then ability to defeat aerodynamic drag. I.e. pointed nose and pointed tail. As a bullets tail has to provide resistance for acceleration during the "interior ballistics" phase, Boat tail bullets represent a proven compromise. If one had a 60 inch barrel, a bullet would work better when pointed at both ends.

3 types of ballistics

1. Interior- what happens before a bullet leaves the barrel.

2. Exterior - what happens to a bullet during atmospheric flight.

3. Terminal - what happend when a bullet hits solid or liquid.

Heck, there are phd's in physics who don't get this. What do you really need to know?

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