The following is a specially-adapted excerpt from the book Handloading For Competition, by Glen Zediker. Visit BuyZedikerBooks.com for more.
by Glen Zediker
A “ballistic coefficient,” or “BC,” is a number that reflects on the aerodynamic performance of a bullet, how well it flies.
I was explaining BC to a fellow once and after talking through all the technical language he said, “So, it’ll hit furtherer on up the hill….” Exactly.
It’s actually a comparison, and that gets explained first. Here’s how it
works: There are “standard” bullets that are mathematical models. Workaday ballisticians know which model to apply to different bullet styles. For most rifle bullets we’ll encounter, one model is a “G1” (there are other models, like G7). The flight of this bullet has been calculated at varying velocities and distances. Pistol bullets, for example, are calculated from or compared to different standard bullet models.
The standard bullet, and, again, let’s say that’s a G1, has a BC of 1.000. An actual bullet that’s compared to the G1 at points, distances downrange, will either be flying faster or slower than the G1. If it’s faster, its BC will be 1.000+; if it’s slower, it will be 1.000- (fractional).
Comparing bullets with different BCs, the one with the higher number will lose less speed over distance. Losing less speed means its flight time will be shorter and it won’t drift and drop as much as a bullet with a lower BC. So, a 0.600 flies better than a 0.550. So: the higher the BC, the less speed lost over distance. That’s it.
Published or stated BCs are either calculated or measured, depending on the maker’s policy. More mathematics than I can wrap my mind around can get these calculations done based on a bullet’s blueprint. Measured BCs involve chronographing at the muzzle and then at other points on downrange, same bullet, same flight. There’s a good question as to which provides the best information. Some logic applied suggests that, without question, a measured BC is more “real world” and therefore more valuable. On the other hand, if the point is to compare bullets, then calculated BCs might be more reliable. One point, however, is that the relationship between measured BCs and calculated BCs is that measured are usually lower…but not always. Reasons for that follow.
All the drift and drop tables (whether printed or digital) you’ll see are based on a bullet’s BC. And, the accuracy of those tables clearly revolves around what the actual BC performance of the bullet you’re shooting is.
So what affects the actual, realized BC of a bullet? A lot of things… Anything that can influence bullet flight influences BC realization. Bullet stability has the lead, though. For a supposed BC to be realized, the bullet has to be “asleep.” If it’s not stable, it’s encountering disruptions that will slow it down. I don’t know many who have had much luck running BC tests “at home.” That’s a logistics issue with chronographs, as could be imagined. Those, however, who have successfully done their own BC testing learn a lot. One, for instance, is that even the rotational speed of a bullet in a test can influence BC. Comparing the same bullet through a 1-8 and a 1-7.5 twist barrel, the 1-8 likely will net a higher BC. The extra revs per second from the faster twist are the likely cause. Easy enough to imagine: 1000-yard BC tests are more revealing than are 500-yard tests.
Atmospherics, which can be a long list of factors, influence BC mightily. Air density is probably the most powerful influence here. Any conditions that allow for easier passage of a bullet through the air don’t detract as much from its BC as any conditions that do serve to impede its flight. A BC, which is based on sea-level air density, can easily show itself as a higher number at 2000 feet above sea level.
Reality is that the demonstrated BC changes from morning to afternoon and day to day and place to place. The calculated BC is not changing, of course, but the mistake is assuming that a BC is some “set” or finite measure of bullet performance. If you’re interested, there’s some valuable information from David Tubb (visit DavidTubb.com). He’s done a volume of work on calculating influences from atmospherics as it applies to his DTR project, which, in one way of seeing it, gets down to understanding why it’s really rare to dial in what a ballistics table says for a particular bullet and speed and distance, and hit the target.
One last bit of information I’ve always found interesting is that a BC is a finite thing, whether the bullet at hand is going to show it or not. Any BC derived from a G1 model, for instance, fits all bullets with that same BC. This was helpful before ballistics apps were as common and easy as they are now. For instance, if there was a new .224-caliber bullet with an advertised BC, but no tables, just find another bullet, of any caliber, with that same BC, plug in the velocity, and the drift and drop figures would be accurate. It doesn’t matter if the other bullet was a .308 or .277 or whatever else.