Measuring BC's

[mtngun]

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..

[mtngun]

Wolf 40gr. 22LR Match Extra from a Ruger 10/22 with a Green Mountain barrel.


That was much better than I expected from the humble 22LR. Note that if you use the uncorrected Chrony velocity (average=959 fps) then the BC is only 0.173, and that might be more accurate considering that the correction correlation does not have data at that velocity, not right now anyway. In any event those numbers ares in line with BC's on the Sierra website, so I'm feeling OK about my measurement technique:

[mtngun]

To save time, I will henceforth calculate the BC based only on the average muzzle velocity and the average downrange velocity, rather than doing the calculation for each individual shot. The resulting BC calculated using the average velocities is usually within 0.001 - 0.003 of the BC calculated using each individual shot. Close enough!

A couple more bullets:


The modest bevel base on this bullet seemed to boost the BC about 0.015 compared to a flat base or gas check.

[mtngun]

30 caliber 124 gr. Loverider.

[mtngun]

[mtngun]

199 gr. 30 Loverider, G1 BC=0.301 when launched at 2227 fps.

[mtngun]

A 203 gr. Loverider scored a low 0.247" BC when launched at 2287 fps from a 14" twist. It seems strange that its BC was lower than the nearly identical 199 gr. Loverider so I'm guessing that the 203 was marginally stable and yawing.

UPDATE: I have since learned that my two chronographs are no longer in sync -- the Shooting Chrony used to read faster than the Caldwell, but when I tested them back to back today, the Shooting Chrony was reading slower, so the old correction factor no longer applies and I'll have to go back to the drawing board.

[mtngun]

This 202 gr. bullet scored BC = 0.228 when launched at 2181 fps from a 14" twist, or BC = 0.207 when launched at 2301 fps. Its SD is about 0.310 so either my downrange chrono is giving bad numbers or else this long bullet is yawing when it launches. It made round holes in paper at 100 yards, but it may have "gone to sleep" by the time it reached the target.

I'll re-calibrate the chrono when time allows and update this post if I find an error.

UPDATE: Yes, I did find an error. The Shooting Chrony used to read faster than the Caldwell, but when I tested them back to back today, the Shooting Chrony was reading slower, so the old correction factor no longer applies and I'll have to go back to the drawing board.

UPDATE 9/1/17: A Caldwell G2 showed up to replace the fussy Shooting Chrony. Since chrono speed may vary with battery voltage, I've resigned myself to checking calibration by shooting back-to-back with the other Caldwell at the beginning of each session. Today the G2 averaged 1.19% slower than the other Caldwell, so I applied that correction factor to the G2's downrange numbers. The result was the 202 gr. Loverider had G1 BC = 0.318 when launched at 2315 fps.

[mtngun]

For what it is worth, the Shooting Chrony was reading 1.2% slower than the Caldwell when I tested them back-to-back today. If I apply that 1.2% correction factor to today's results then I got:

30 cal 202 gr. Loverider spitzer = 0.297 G1 BC @ 2300 fps muzzle

30 cal 193 gr. Loverider spitzer = 0.260 G1 BC at 2278 fps muzzle (it had tested at 0.301 BC on August 18)

30 cal 203 gr. with point filed flat = 0.281" G1 BC at 2265 fps muzzle (about 0.020 less than the original version)


Now I have to decide what to do about my untrustworthy chronograph. I wonder if the drift is due to the battery voltage? The Caldwell runs off an AC adapter, but the Chrony has to run off a battery since it downrange where there's no power. At any rate, the Chrony seems to drift.

Options:
-- recalibrate the Chrony every day that I measure BC. Ugh! I don't like that option, but it may be prudent.
-- replace the Chrony with another Caldwell. The Caldwell has a faster clock and a bigger sensor window. However, the downrange Caldwell would have to run on a battery, so it might drift with battery voltage???
-- replace the Chrony with a LabRadar. However, I doubt if the LabRadar would be able to "see" through my muffler, so I would only be able to use it on the outdoor 200 yard bench, which is not nearly as convenient to use as my indoor 100 yard bench.
-- if money were no object I would get one of each -- a 2nd Caldwell for the 100 yard range, and a LabRadar for the 200 yard range. Maybe someday ...