Arthur Schopenhauer said:
"All truth passes thru 3 stages. First, it is ridiculed. Second, it is violently opposed. Third, it's accepted as being self evident."
I think we're somewhere between step one and two with properly referencing BC's.
There are those 'advanced users' who are tuned in and get it, but we all agree it will take a long time before the practice is common from the top down.
We did some BC testing at the Ben Avery range in Phoenix last week at 1000 yards. Mid Tompkins has a chronograph with 4' hardware (not optical) screens that can be hit and measure velocity at long range. Just for kicks, I brought along my acoustic sensors which measure time of flight. It was interesting to see how the BC's derived from the velocity data compared to the BC's derived from the TOF data.
Eric and Michelle did the shooting, I monitored the acoustics and MV, and Mid ran the pits and handled the downrange chrono. It was quite a party
Here are some interesting conclusions/results from the testing:
1) The G7 BC's derived from the velocity data were very similar to the G7 BC's derived from the TOF data. However, the G1 BC's were vastly different when derived from velocity as compared to TOF. The practical consequence of this fact is that if you're out to 'measure' the G1 BC of your long range bullet, you can arrive at a different number depending on how you measure it (velocity drop or tof). But if you reference the BC to a more appropriate standard, it doesn't matter what metric you measure, you will get a 'true' value that applies for all performance metrics.
2) The bullets that were shaped more like the G7 standard projectile (VLD's, .308 155 grain Lapua Scenar, .308 155 grain HBC (Australian made)) had the least difference in their G7 BC's, and maximum difference in their G1 BC's when derived from the different methods. Other bullets that have tangent ogives still matched better for G7 than G1, but a different model would be required to make a better match.
As an example, the Lapua 155 Scenar showed a G7 BC of .233 derived from the velocity data, and a G7 BC of .234 based on the MV and TOF. The difference is only 0.4%. The G1 BC was .453 based on the velocity data and .465 based on the MV and TOF data, a difference of 2.6% which is 'significant' in terms of predicted drop at long range. The Berger 155.5 FULLBORE bullet (a tangent ogive design) showed 2.2% difference for G7, and 4.8% difference for G1. I would bet that referencing the BC to the G5 standard would result in less than 2.2% difference because this bullet more closely resembles G5.
The point is that if you don't use the right standard drag model, you can arrive at drastically different results for BC based on how you measure it.
3) I was pleased to see that 8 out of 17 of the bullets tested matched my previously measured G7 BC's within 1%. All but 3 of the 17 matched within 2%. Achieving this kind of agreement despite the fact that we tested different lots of bullets from different rifles and different average flight velocities was very encouraging.
Regardless of how 'dug in' some of the others in the industry may be, the fact is that we're at the point in the shooting sports where shooters recognize the importance of good information. They're going to get it one way or another, it's just a question of how much trouble it's going to be. Lapua stepped up by providing the radar measured drag curves unique for each bullet which is golden information, but is not a practical option for most companies due to cost. Berger publishes G7 BC's. Sierra gives multiple G1's. All the others aren't yet offering anything but the G1 referenced BCs. Of course my book has G7 BC's for many LR bullets but face it, only a small fraction of shooters will know about and use that data compared to those who look to the bullet companies for BC's.
Jeff makes a good point about considering other standards in addition to G7. That point has been illustrated by much of my testing which shows an imperfect match of drag data to the G7 model, especially for tangent ogive bullets. There are a couple reasons why I continue to advocate the exclusive use of G7 referenced BC's including:
1) By using one standard for the LR 'class' of bullets, you maintain the ability to compare bullets by BC. If you don't use the same reference, you can't compare. For example, what's better, a G5 BC of .354, or a G7 BC of .345? I feel that the small practical difference in 'match' between BC's referenced to G5 vs G7 is not worth sacrificing the ability to compare bullets based on a BC that are all referenced to a common standard.
2) It's very difficult to measure with certainty which model a bullet matches best. It's clear that all long range bullets match G7 better than G1, but when you talk about G7 vs G5, the difference is more subtle. You might be able to do this with certainty after carefully testing the same bullet many times, but it's not a practical thing to consider applying to many bullets. I typically use 4 acoustic sensors spaced in 200 yard intervals from the muzzle to 600 yards and most of my data isn't good enough to distinguish between a G7 or G5 match
with certainty. The biggest difference in the shapes of the standard drag models is near the transonic speeds where the flight quality of the bullets becomes questionable and 'rifle dependant' because of stability considerations.
If you're an advanced user like Jeff and do the testing to demonstrate that one drag model is decidedly better than another for a particular bullet, then you are correct to use that standard. However, because of the considerations raised above, I am of the opinion that opening the shooting world up to all the different standards as a common practice is not the right thing to do. Can you imagine a different standard drag model for each bullet type being invented? You would have the Accubond drag model, the SMK drag model, the Berger VLD drag model, the Tripple Shock drag model, the Grand Slam drag model, the Scenar drag model, the Vmax and Amax drag model, etc etc. And even then, they would still be a compromise for some of the bullets in their own line! The 178 AMax has a different ogive and boat tail than the 208 AMax. The 210 SMK is quite a bit different in shape than the 220 SMK. I truly believe that there is a lot of value in simplicity and limiting the scope of standard drag models to G7 for the LR class of bullets is the right thing to do
for the general public.
Everyone stay warm and enjoy the holidays.
-Bryan
