Affordable Method of BC Measurement Over Long Ranges

[nralifer]

I would like to share a method we have been using to measure ballistic coefficients (BC) of not only commercially available bullets but also bullets we are developing. The materials needed should be readily available to anyone doing long range shooting and reading this forum. These include a personal computer with a built in microphone, downloaded Audacity software that records a sound wave form (this is a free download), two Meridian walkie talkies (about $50), a steel gong, a chronograph, access to the JBM ballistics web site, a laser range finder, a cardboard box lined with foam that will fit your computer, a cell phone, the Weather Channel app on the phone, and a place to shoot. We have used this method to measure the BC of Berger bullets and found that our measurements are within less than 5% the BCs published by Berger on their website. We measure the time of flight by measuring the time elapsed between the muzzle blast and the bullet impact on the gong, and do this over 400-650 yds. The method works best in calm conditions but will work in winds up to 20mph. Beyond that wind calls become a problem.

 

To do this one places one of the WTs in transmitting mode behind the gong about 2 feet in such a way that it will not be struck by the bullet or bullet splatter. The other WT is in receiving mode at the computer whit itself is within the foam lined box to not lay shade the monitor from the sun, but also to reduce background noise. The computer microphone should be about 3 feet away from the muzzle. With the Audacity software recording, a shot is taken, the bullet hits the gong, and the noise of the impact is transmitted back to the receiving WT by the transmitting WT at the speed of light. One then can announce verbally what the MV was for the shot and the bullet type so that the information is recorded on the same tracing as the shot. The Weather Channel app is used to record weather conditions, or a Kestrel device can be used.

 

The picture shows a close up of the waveform tracing as it appears on the monitor. From Lt to RT there are three main spikes. The first is the muzzle blast, the second the gong impact as recorded from the transmitting WT transmitted at the speed of light and the third is the gong impact as heard by the computer microphone as the sound comes back from the gong through the air at the speed of sound. The shooter will gear two gong impacts as well. The strength of this method is that it will give reliable measurements in windy conditions since the transmission of the transmitting WT is not significantly affected by the wind.

 

Although the maximum distance we have recorded BCs has been 650 yds, theoretically this could work over 1000 yds. The limiting factor is flight time since the JBM calculator only allows flight times of under 1 second. 400-500 yds seems work the best since gong hits are reliable in windy conditions (a big factor in South Dakota). This method tends to underestimate by less than 5% the published Berger BCs. We think it works well as a method of comparing bullets relative BCs. and to see if those BCs published by other companies are inflated.

[ThomC]

It might be easier to learn to hunt.

[Big Browns]

That's great info! Thanks for sharing.

[bonecollector777]

I'm just impressed you figured all that out with all those different materials. Interesting stuff.

[nralifer]

It's a good way to check the published BCs. You can believe the published BC if you are able to measure within 5 % of what they claim. Also you need at least 5 good shots and take the average BC of the 5. 400-600 yds is about the best distance to use. You can get hits on the gong reliably, and the bullet flight times are under 1 second. Can't wait to test the ELD bullets against the Bergers and our own.

[goatsie]

That is the same concept that Brian Litz uses to confirm BC. That is a load of work! Using bullets with unknown BC this is a good method. Thanks heavens I use JLK and Begers with very accurate BCs provided by Litz!

[nralifer]

It really is not hard to set up. If one goes to shoot long range with a gong adding the transmitting WT 2 feet behind the gong and bringing one,s computer is no big deal. The other day we set up, collected data on 30 shots on 3 different bullets at 419 yds and took everything down in 1.5 hrs. The time consuming part is reading the shot wave forms and calculating the BCs.

[nralifer]

We had the opportunity to do some bullet testing this yesterday. Tested three bullets we designed against the standard, the 185 gr Berger Juggernaut. Used a 308 Win and Oehler 35 chrono. Measured BCs over 654 yds. Wind was marginal at 6-15 mph, mostly 12mph mostly straight on but as time went on became gusty and would shift directions. The bullets we made are all copper hollow points tipped with aluminum tips (see pics on "Berger Failed Me Horribly"). We call them Bulldozers (BD). Results are as follows:

 

150 BD BC of 0.480

180 BD BC of 0.540

196 BD BC of 0.575

185 Berger BC of 0.542 (advertised BC of 0.555)

 

We used the BC measuring method as described above

 

For the Berger bullet the difference in BC measured vs advertised is 0.013, or 2.3% of advertised. We see this consistently, so we have confidence that this method is fairly accurate and is not overestimating the BC. The figures quoted are the BCs actually measured and not corrected. We are happy with the 150 and 180 bullets, and a little disappointed about the 196 bullet. Will be making some changes to the design to see if we can't get to measured 0.6 BC. One thing though, we think our bullets beat all the other all copper hunting bullets in the same or similar weight category as far a BC is concerned ( Hornady GMX, Barnes LRX and Nosler Etip) Of note is that I cleaned my barrel prior to the test after about 500+ shots using our bullets and those of other manufacturers, and found there was no copper fouling. The barrel was made by Barlein, has a 9.5 twist with 5R rifling, and is 24 in. long. Shoots 1/4 to 1/2 in. groups with any of the tested bullets and is in a Savage 10 action. In the 308 Win I have been using exclusively Hodgdon Leverevolution powder and WLRM primers.