KEEN rifle shooters have a strong interest in a number called the ballistic coefficient, which offers some highly useful information.
But there's a catch. Ballistic coefficients are often misunderstood - and misunderstanding can trap shooters into making bad choices, such as selecting the wrong bullet or powder charge. A bit of explanation will not go astray.
When a bullet leaves the barrel, it tries to follow the line of the bore. Gravity, however, pulls the bullet earthwards and air resistance slows the bullet down. The result is a curving flight path or trajectory, and the curve steepens with increasing range.
Now, gravity has the same effect on all bullets. But the bullets themselves differ: some are heavier or better streamlined than others, so they don't lose speed as quickly.
The extent to which a projectile is able to conserve its speed is called its ballistic efficiency.
A bullet with high ballistic efficiency will cover a given distance faster than one with lower ballistic efficiency, if both are launched at the same velocity.
Consequently, because gravity has less time in which to act, the more efficient projectile will have a flatter trajectory. Where it weighs the same as its rival, it will also have more energy and momentum when it reaches the target.
The most useful indicator of ballistic efficiency is the ballistic coefficient, whose symbol is C (not BC).
It is a number (more correctly, a value) which reflects the shape, weight and cross-sectional area of the projectile. Higher numbers mean better in-flight performance. A bullet with a C of .351, for example, is less affected by air resistance than one whose C is .289.
Most manufacturers quote C ratings for their bullets. Unfortunately, the methods used for deriving these coefficients vary from maker to maker - some work from actual test firings while others rely on analysis of the bullet's shape, size and weight.
Another complication: the value of C depends partly on velocity. Let's look closely at two different projectiles:
Take Sierra's 300-grain FNHP in calibre .458. At 1000 to 1200 feet per second, its C is .240. Above 2200 fps, its C drops to .126 - a vast change.
The same maker's .243 100gn SBT goes the other way. Under 1800 fps its C is .391 but rises to .445 above 2800 fps.
In short, that often-quoted rule "The higher the velocity, the lower the ballistic coefficient for a given projectile" is on shaky ground. Though it holds true for the majority, there are many exceptions.
Regrettably, some makers won't release the velocity data. However, some of the exterior ballistics programs available for home computers can calculate C values. Finally, in many field-shooting and target-shooting situations, the wind can spoil shots by blowing bullets off course. Though we must always allow for the wind, bullets less affected by the wind are an advantage.
Here's the crunch: higher C ratings imply less wind deflection, but the connection between C and wind speed is not as simple as it looks at first. We will have to leave that subject for another time.
