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Basically, it's the SHAPE and DENSITY of the bullet. Most bullets are of fairly similar density, so the basis for comparison among bullets usually turns on their SHAPE, if I'm not mistaken. The BC is a measure of the bullet's ability to "cut through the air" with minimal drag; thus the higher the BC, the less bullet "drop" at the same distance out with the same muzzle velocity. So a bullet with higher BC has about the same drop at long distances as a bullet with a little more velocity but less BC coefficient. Basically, certain ratios of length to width make for better BCs regardless of the absolute size of the bullet. For example, 7mm (.284) bullets have the best BC in the 145-165 grain range, but .308 bullets have other widths beat in the 170-200 grain range, and the .338s have the best BC with about 250 grains. And, in addition to the general width to length ratio, certain specific bullet charactistics increase BC, though a little less important than general length to width ratio, I think. For example, the sharper the point of the bullet nose, the higher the BC (just makes sense, a point cuts through air better than a round nose). Also, boat tails have a slight edge over flat base. and the actual rate of change of the curves from the flat sides of the bullet near the base as it goes toward the point is important too. Though I'm not sure what kind of curve is best - I'm thinking slowly rounded is better than distinct angle. So, clear as mud?
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