bullet metric question
In my Hornady reloading manual, it lists a bullet metric labeled as "B.C." What does that measurement mean?

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Yp. Now in several manuals, if you go all the way to the back, and there is downrange data, broken out by BC. Gives you the retained energy and velcity per grain for different BCs starting at different speeds.

A "metric"? Doesn't that refer to the metric measurement system; meters, centimeters, millimeters, liters, etc.?

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'B.C." is short for Ballistic Coefficient. Think of the projectile being a tire and the air as a roadway. A higher "B.C." would be the equivalency of a harder and or smoother road surface. A tire will role farther and easier on smooth pavement than on/in soft sand or loose gravel. This analogy is correct only in the common effects. With bullets, the shape of the nose and heal with the amount of mass (weight) enclosed in the air foil disruption caused by the motion of the bullet (at its present speed and that changes) are used to give the coefficient number. Closer to '1', the less loss of energy/velocity of the projectile due to the effects of the bullet's design. Short range uses, Pistols, shot guns and the like, are effected by B.C. but the ranges are comparatively so short as to not be much of a factor. Longer range uses and the B.C. becomes much more apparent. I seldom get to shoot much more than a hundred yards (trees and brush), so I seldom worry with B.C.. Yes the effects are their, but I am more of a factor. Hope this helped a little. 
Quote from Wikipedia
"In ballistics, the ballistic coefficient (BC) of a body is a measure of its ability to overcome air resistance in flight.[1] It is inversely proportional to the negative acceleration —a high number indicates a low negative acceleration. BC is a function of mass, diameter, and drag coefficient. It is given by the mass of the object divided by the diameter squared that it presents to the airflow divided by a dimensionless constant i that relates to the aerodynamics of its shape. Ballistic coefficient has units of lb/in˛ or kg/m˛. BCs for bullets are normally stated in lb/in˛ by their manufacturers without referring to this unit." And also from Wikipedia "The formula for calculating the ballistic coefficient for bullets only is as follows:[2][3] where: BCBullets = ballistic coefficient SD = sectional density, SD = mass of bullet in pounds or kilograms divided by its caliber squared in inches or meters; units are lb/in2 or kg/m2. i = form factor, i = ; (CG ~ 0.5191) CB = Drag coefficient of the bullet CG = Drag coefficient of the G1 model bullet M = Mass of object, lb or kg d = diameter of the object, in or m" 
Metric defined....
Here is the dictionary definition of "metric" as I used it in my question:
A metric is any type of measurement used to gauge some quantifiable component, including measurement of a company's performance, return on investment ( ROI), or any numeric comparison... 
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