### Measuring BC's

Posted:

**Wed Jun 14, 2017 10:02 pm**Let's measure some BC's. I used two chronographs, one placed 5 yards from the muzzle and the other 99.5 yards. But first, I "calibrated" two chronographs by placing them back to back and comparing over a range of velocities.

A Caldwell chrono is just inside the green muffler box while an older Shooting Chrony stands 4 feet downrange from the Caldwell.

The raw data from the back-to-back test. The Chrony reads the same as the Caldwell at low velocities (a standard velocty 22LR) but at higher velocities, the Chrony gives faster numbers.

Which chronograph is right? The Caldwell is a newer model with a faster clock so I'm going to assume that it's more accurate than the Chrony, and I will apply a correction factor to the Chrony results to make them equivalent to Caldwell results. A spreadsheet crunched the numbers. However, the resulting correlation equation was sort of backwards from what I wanted, converting Caldwell results to Chrony results when I wanted to do the reverse. A little bit of junior high algebra rearranged the equation to:

Corrected Chrony = 0.9643 X Chrony + 38.9

Then I placed the Chrony at the 100 yard target. The Chrony sensors have a very small "window" where they will read reliably, so quite a bit of fiddling was required to place the Chrony in exactly the right spot and sometimes several shots were missed before I got it working reliably. But at least I did not shoot the chronograph!

I used JBM's online calculator to crunch the velocity numbers. Here's a sample calculation. The pressure, temperature, and altitude settings make a noticeable difference, so take care with those. I went with the G1 coefficient since these ogives are closer to a G1 ogive.

Here's the data for a 30 caliber 186 gr. "aggressive" loverider (ALR) with a sorta tangential ogive.

A 30 caliber 190 gr. "ugly" loverider (ULR) with a sorta secant ogive.

A 30 caliber 182 gr. "coned" loverider (CLR) with a cone ogive. UPDATE: I found an error in the spreadsheet and the correct BC is 0.254, not 0.250.

These are the 3 bullets.

That's all for now. I'll update this thread as more data rolls in for different bullet designs, different calibers, etc..

A Caldwell chrono is just inside the green muffler box while an older Shooting Chrony stands 4 feet downrange from the Caldwell.

The raw data from the back-to-back test. The Chrony reads the same as the Caldwell at low velocities (a standard velocty 22LR) but at higher velocities, the Chrony gives faster numbers.

Which chronograph is right? The Caldwell is a newer model with a faster clock so I'm going to assume that it's more accurate than the Chrony, and I will apply a correction factor to the Chrony results to make them equivalent to Caldwell results. A spreadsheet crunched the numbers. However, the resulting correlation equation was sort of backwards from what I wanted, converting Caldwell results to Chrony results when I wanted to do the reverse. A little bit of junior high algebra rearranged the equation to:

Corrected Chrony = 0.9643 X Chrony + 38.9

Then I placed the Chrony at the 100 yard target. The Chrony sensors have a very small "window" where they will read reliably, so quite a bit of fiddling was required to place the Chrony in exactly the right spot and sometimes several shots were missed before I got it working reliably. But at least I did not shoot the chronograph!

I used JBM's online calculator to crunch the velocity numbers. Here's a sample calculation. The pressure, temperature, and altitude settings make a noticeable difference, so take care with those. I went with the G1 coefficient since these ogives are closer to a G1 ogive.

Here's the data for a 30 caliber 186 gr. "aggressive" loverider (ALR) with a sorta tangential ogive.

A 30 caliber 190 gr. "ugly" loverider (ULR) with a sorta secant ogive.

A 30 caliber 182 gr. "coned" loverider (CLR) with a cone ogive. UPDATE: I found an error in the spreadsheet and the correct BC is 0.254, not 0.250.

These are the 3 bullets.

That's all for now. I'll update this thread as more data rolls in for different bullet designs, different calibers, etc..