Originally Posted by
Big Z
Now, we all know that a feather is going to drop to the ground slower than a lead ball because it's much less dense and is influenced by air much more. However, I'm wondering why there seems to be nothing to figure the difference in flight between, say, a plain copper bullet and a bullet with a lead core (let's ignore the fact that the copper bullets would have to have more volume to retain the same mass). The lead core one would have more density, and this would lead me to believe that it would be affected by wind less.
However, ballistic coefficients are only calculated by shape, the relative angles of a certain bullet etc and this is what we use in calculators. What are standard projectiles made out of? Is compensation consistently accurate by adjusting BCs in calculators? I'm delving into the world of ballistics and some things just require good solid experience to answer
Not exactly true. The BC of a bullet can be ESTIMATED by looking at the form factor of the projectile, sectional density (the ratio of mass to cross sectional surface area) and the approximate velocity the bullet will be fired at, and comparing it to a similar bullet with an experimentally derived BC. Many bullet makers use calculations to approximate their bullets BC's because they know that a) most hunters never shoot far enough for BC to matter, but like the "bling factor" of a bullet with a high BC, b) they can get away with fudging the numbers to get a higher BC value than the bullet would have if actually tested, which sells more bullets (see "a") and c) it's a heck of a lot faster and cheaper to make an educated guess than to have someone actually carefully load and fire a few hundred rounds through chronographs at a variety of velocities and distances, then empirically calculate the BC based on real world bullet performance.
So what, exactly, is BC? Well, quite simply its the ratio of the velocity loss rate of a given projectile compared to a "standard projectile" fired at the same velocity. The G1 standard projectile is the one used by most manufacturers because it gives the highest BC numbers, but is also the LEAST representative drag model for modern spitzer boat-tail rifle bullets.
The G1 projectile
The caliber and shape of the bullet don't directly effect wind drift. The important factor is time of flight. If you compare a .308cal 180 grain bullet with a BC of .450 fired at 3000fps to a 100gr .243" VLD bullet with a .450 BC also fired at 3000fps, the two will have virtually IDENTICAL wind drift. Why? Because they are fired at the same velocity and lose that velocity at the same rate, so the time of flight to any given distance is the same.
Mike