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Posted: 11/21/2001 3:12:17 AM EST
Whats the definition and what variables do you need to calculate it? Thanks Guys!



Don't hold me to this, but it is my understanding that it calculated by using the shape and length of the bullet.



Ballistics is off in a corner by itself when it comes to aerodynamic calculations. None of the usual coefficiens of drag or lift are used in ballistics. It's my understanding that sometime in the beginning of all things ballistic, a "standard" projectice was fired & measured. All others since then are related to the original standard.
IOW, it's basically measured, not calculated. There are charts for estimating BC by similarity to other, already measured, projectiles. There are several good websites with this information, none of which come to mind just now. Try a google.com search on "ballistic table coefficient" or something similar. 




Life Member NRA, TSRA, LEAA, LSBA

B.C. represents the aerodynamic characteristics of a projectile in flight, specifically with regards to loss of velocity at distance. The BCs you see quoted by Sierra and other manufacturers are actually calculated estimates (and usually damn good ones) based on a bullet's shape compared to known models, among other characteristics. The best but most impractical calculation of BC is to shoot the bullet across a chronograph at the muzzle and at the target; the practical considerations are obvious at long ranges. You can then average the velocity loss at a given distance and calculate actual B.C. using relatively simple algebraic equations outlined in Sierra's loading manual. Personally, I can't track all the numbers, and ballisticians universally seem very distracted and aloof, so I just use my Sierra program it leave the math stuff to smarter people. B.C. is so important because it allow prediction of how fast a bullet moves downrange, hence how long it takes a bullet to move downrange (time of flight). This time of flight is important in calculating the effects of outside forces such as gravity and wind. The longer gravity acts on a bullet, the more it drops; hence, a projectile that takes longer to travel 100 yards will be under the effect of gravity for a longer time and therefore drop more compared to a faster projectile. Ditto for wind drift; this poses the seemingly unnatural phenomenon where a lighter bullet, say a 77 grain .223 Sierra MatchKing, has no more drop or wind drift than a heavier bullet weighing 168 grains such a 30 caliber .308 Sierra Matchking when fired at comparable velocities because they share similar B.C.s. I still haven't come to grips with that one, but that's what the math says... shooter 

"Shut up and shoot, son...I'm gettin' old here."

Think of BC as a measure of the bullet's aerodynamic drag  i.e., a measure of how fast the bullet will slow down s it moves down range, and a measure of how flat the trajectory is with respect to bullets of the same weight, flying at the same speed, but different BC.
Since a high BC, low drag bullet loses speed slower than a low BC bullet, it arrives at the target quicker (and with a higher percentage of its muzzle velocity remaining), and therefore it is exposed to cross wind deflection for a shorter period of time than high drag bullets, so the actual deflection in a cross wind is less for high BC bullets. Sierra measures BC in their underground tunnel. Unfortunately it's usable length is too short to gather BC data at very long ranges, over 200 yards or so (I forget the exact distance they are using). Many other shooters measure BC at regular shooting ranges using two chronographs to measure the bullet speed at the muzzle and at the target (usually at the target). Sierra has a store at the plant where seconds can be bought by the pound, and they also give palnt tours. Starline Brass is next door. 


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