It’s not about how fast a bullet is but how fast it slows down.

“How fast a bullet slows down is a function of its drag,” said Jayden Quinlan, Hornady® senior ballistician. “Bullet drag defines the things that we mainly care about in external ballistics.”

The greater a bullet’s drag, the longer it takes getting to its target and the more it can be impacted by external forces. The major concern is the impact to elevation, which affects a bullet’s vertical path; and windage, which impacts it horizontally.

“When we’re talking about a bullet, the more feet per second we can retain per yard, the better it is,” Quinlan said. “That means it’s slowing down at a less rapid rate, gets to the target faster, has less drop due to gravity and less deflection due to wind.”

Less drop and deflection mean more accuracy.

“We want to be able to predict how fast or how much a bullet is going to drop due to gravity or deflect due to wind,” he said.

The first aspect that affects a bullet’s drag is its shape. A race car, for example, has much less drag than a bread truck.

With the development of chronographs and acoustic targets, ballisticians could determine the velocity of bullets of different shapes and how much velocity changed downrange.

“If you can measure the velocity change,” Quinlan said, “you can measure the drag.”

Measuring velocity with a chronograph or acoustic target provides one data point, like a snapshot, for one set of instrumentation. This leaves the majority of the bullet’s flight unmeasured.

“You have to assume what the drag of the bullet is before you measured it, between where you measured it and after where you measured it,” he said. Trying to measure the velocity of the bullet at hundreds of points downrange is laborious and expensive using these methods.

That’s where ballistic coefficient (BC) comes in.

“It’s a scaled value that is used to estimate how fast a bullet slows down,” Quinlan said.

Most BC-based calculators use two standard bullet shapes to determine drag curves for trajectory projections. G1 has a flat base and a stumpy ogive. G7 has a boattail and a sleek ogive.

“When we’re talking about BC, we say you look like this, so you slow down like that,” he said.

But many modern bullets, such as the A-Tip® Match, may not match the standard shapes sufficiently enough.

“The A-Tip® Match is way more race car-like than that G7 standard,” Quinlan said. “It’s not going to slow down the same way.”

Shooting a bullet under Doppler radar can reveal much more about its actual drag curve.

“This bullet was tracked for 1,510 yards over nearly 55,000 points,” he said, citing one example. “That math comes out pretty easily to one data point per inch.”

But a BC calculator doesn’t know your actual bullet’s drag curve. It only knows the G1 or G7 standard.

“Inside a BC-based ballistics program, there’s the relationship between what the program knows and what’s real but the program does not know,” Quinlan said. “The drag curve shape of your bullet is probably not going to be the exact same drag curve shape of the G7 bullet, and any mismatch in shapes is going to result in a prediction error by the program.”

Quinlan gave an example in which the difference between the standard curve and the actual curve resulted in a 3-foot error at 1,500 yards. In effort to correct the error, the shooter changed the BC in his ballistic calculator, called truing.

“We said that error wasn’t acceptable, so we changed the BC to try to clean that up a bit,” he said. “We cleaned it up to where now we’re only off by 3 inches at 1,500.”

But changing that BC widened the gaps between the standard curve and the actual curve further out and early on.

“No matter what we did, we couldn’t really fix it. We could find a sweet spot where we normalized the errors that were there, but we still had errors,” Quinlan said. “The reason that happens is because you can’t bend this curve in the program.”

“It can move up and down to try to match what BC you tell it, but you can’t put a knee in it,” said Seth Swerczek, Hornady® marketing communications manager.

“That’s how almost all of us started shooting stuff far away, taking a number that we got from a bullet box, putting it into the calculator, hitting go, and you go figure it out,” he said. “You true it up as best you can, not knowing that you are creating other errors elsewhere every time you try to true it at a given distance.”

Unlike BC-based calculators, the Hornady Ballistic Calculator with 4DOF® is based on drag coefficient and aerodynamic moments and coefficients that calculate a bullet’s dynamic responses in flight.

“We fire those bullets over the Doppler radar, and we record how fast they slow down. Then we do it over and over and over again, and we take an average of all those,” Quinlan said. “That’s what you’re using when you use the 4DOF® program to calculate how much velocity you’re going to retain.”

Other techniques of “fixing” a BC-based calculation, such as truing muzzle velocity don’t address the actual problem. Measuring muzzle velocity with a quality device and then immediately having to lie to the program to get it to match what is observed means something is wrong. In this case, it is that the drag being used by the program to predict how fast your bullet slows down does not match how fast your actual bullet slows down. Changing muzzle

“It totally depends on that bullet’s unique drag curve,” Quinlan said, “and you have no way to know what that is unless you have a Doppler radar that can track that bullet through its whole flight.”

The Hornady Ballistic Calculator with 4DOF® represents a better way, and it doesn’t cost anything.

“We have a product, and we want you to use it,” Swerczek said. “We have the technology now to have more first-round impacts, to have better solutions.”

Learn more

To learn more about bullet drag and ballistic coefficients, tune into the Hornady Podcast, episodes #034 and #044.

Download the free 4DOF® App.