Home Forums Hunting & Shooting Ask The Gunwriters Oehler System '89 (Measuring BC)

The longer the distance the BC is measured the more accurate the BC

I've wondered the same thing. Despite shooting a bunch of shots, I have no real conclusion yet. In other words, BC doesn't track velocity. Some shots have low velocity and high BC while some have high velocity and low BC.

Back in the day, I decided I knew what was causing the gun-to-gun BC variance. I just knew it was the crown. So, we started with a good crown and then messed it up. Not only did we prove I didn't have the answer, the damaged crown shot a better group than the "perfect" crown. One of my guys wrote it up in an article that was printed in Precision Shooting. You can read it here: https://www.longrangehunting.com/articles/the-over-rated-crown.670/

My opinion (note I say "opinion") is that many factors affect drag. Twist rate, projectile design, launch dynamics, damage to projectile tips during recoil (rounds in magazine), density of the air you are shooting in, etc.

While I certainly want to know the "why", at this point I've been more concerned with the ability to measure and sort to find what will work best for those I'm associated with. I'm a big believer in what President Regan said: "Trust but verify".

We are in the infancy of TOF measurements being economically available. The more you conduct TOF measurements, the more questions you have.

I'm excited about the questions that will surface when there are 100 people with this capability. I'm even more excited about what will be asked when 500 or more have this capability.

Also, think of what this will do for ammunition manufacturers. Way back, before chronographs, nobody really cared how consistent the velocities were (The SAAMI standard used to be +/- 90 fps). When chronographs became common, the ammunition manufacturers had to pay more attention to consistency.

That is where we are on drag (BC). Once consumers have the ability to measure it, the manufacturers will likely either improve the consistency or face the wrath of internet posters proclaiming how horrible it is vs. one they found that is great.

Yeah, I'm excited. I've been heavily involved in ballistic testing since 1997. Before that, I was a handloader and avid shooter. Perhaps I'm a bit of a ballistics nerd but, hey, everyone has to have something they are interested in.

Maybe you can answer this question. Many bullets are made in forming dies and with use they wear producing longer olives. Bullets made on a screw machine seems would be more identical and have a more consistent BC. Have you found this to be true?

Thanks for your input, Buford. Interesting results.

We know that the drag coefficient of a projectile does depend on velocity, but the nature of that dependence changes with the operating regime, whether sub-sonic or super-sonic. For small variations in velocity, there are other factors that likely dominate, such as the gyroscopic stability of the projectile if that stability is marginal or nearly unstable. Once the SG factor gets up around 1.5, additional rotational speed does not seem to significantly change BC. Factors like tip damage and rifling engraving change the form factor of the bullet, and clearly change the BC of the bullet compared to the undamaged/unfired state. Transient stability behaviour as the bullet leaves the muzzle also affects the drag coefficient. Drag does, indeed, depend on air density, but I would suggest that this is an environmental/external factor and not related to the drag coefficient of the bullet itself.

BC is just a selling point till you go past 300 yards.

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Maybe you can answer this question. Many bullets are made in forming dies and with use they wear producing longer olives. Bullets made on a screw machine seems would be more identical and have a more consistent BC. Have you found this to be true?



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The worst ES of BC I've ever seen was on some lathe turned bullets. It had an average G7 of .397. In a 10-round test, the low BC was .379. The high was .412. How the hell are you supposed to hit anything with that bullet at really long ranges?

I've also tested some lathe turned bullets that had low BC variance.

Why? I don't know why. Yeah, I'd like to know why.

Only way to know how a bullet performs is to test it.

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Thanks for your input, Buford. Interesting results.

We know that the drag coefficient of a projectile does depend on velocity, but the nature of that dependence changes with the operating regime, whether sub-sonic or super-sonic. For small variations in velocity, there are other factors that likely dominate, such as the gyroscopic stability of the projectile if that stability is marginal or nearly unstable. Once the SG factor gets up around 1.5, additional rotational speed does not seem to significantly change BC. Factors like tip damage and rifling engraving change the form factor of the bullet, and clearly change the BC of the bullet compared to the undamaged/unfired state. Transient stability behaviour as the bullet leaves the muzzle also affects the drag coefficient. Drag does, indeed, depend on air density, but I would suggest that this is an environmental/external factor and not related to the drag coefficient of the bullet itself.[/quote]

That has not been my experience in all bullets. Some benefit from higher SG than others. Additionally, velocity only plays a part if the form factor of the bullet is not a match to the standard you are comparing it to.

For example, I've tested bullets that showed a different BC number at two ranges if figured on the G7 curve BUT showed the same number at the same ranges if figured on the G5 curve.

Thicker air requires more spin to be stable. That is why I mentioned it.

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BC is just a selling point till you go past 300 yards.
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True. I wish I had said it so simply!

Here is another good example of BC not changing IF the system matches the drag model used.

I tested TOF of Fiocchi .22LR Subsonic HP ammunition at three distances, 100, 200 and 500 yards (rounded to nearest 100 yds). Keep in mind that the G1 and G7 numbers are the same shots, just using velocity, atmospherics and distance to calculate it to each standard. That is one of the great things about this system. Although you have to choose a drag function under which to test, you can replay the shots under other drag functions without re-shooting.

Here are the results:

........G1 ... G7
100 .113 .051
200 .108 .051
500 .097 .051

So, the G1 BC changed with velocity but the G7 didn't.

What does that tell me? That the G7 was a perfect fit for this load, in my rifle, despite the shape of the G7 standard not being anything close to the shape of the bullet I was shooting.

Yes, it’s true that some bullet forms require higher SG than others to rapidly damp the transient pitch and yaw as the bullet leaves the muzzle. The SG=1.5 figure is just a rough generalization.

Note that I mentioned velocity versus drag, not BC. Drag always depends on velocity, but as you pointed out there are various standard form factor models which model the drag-velocity relationship for different form factors. If your projectile’s form factor perfectly matches the form factor of the standard model, then the model will correctly predict drag through the velocity curve, and the BC will remain constant for a given velocity regime (sub-sonic, trans-sonic, super-sonic). The drag relative to a model remains constant, but that doesn’t mean that the bullet’s absolute drag remains constant.

Again, thanks for reporting your results. Very interesting!

Dr. Oehler is amazing !! I talked to him years as I tried to understand 22 Rimfire problems. I thought & still believe that bullet wobble affects rimfire flight & varies some from lot to lot & bullet to bullet. T

I think you could look at the Standard Deviation of BC to explore your theory. "Wobbling" bullets would have more drag and, I'd think more inconsistent drag.

I've got one of those 1:9 barrels for a 10-22. I plan to do some testing to compare to a 1:16 barrel.

Circling back to see if anyone else has begun to measure their BC numbers.

Since I first understood that BC shot to shot variability is similar to velocity shot to shot variability I've thought it something important for long range shooters to measure.

Bryan Litz has done a wonderful job of measuring actual performance for shooters at those events where AB radar has been present. That radar can't be everywhere, though. Like Parshal said, if you don't have regular access to AB doppler, the '89 is the next best thing.

I've run an '89 right next to the AB radar, measuring the same shots. The numbers agreed as much as you could expect two radars to agree.

I also posted the above in the Long Range Hunting forum. Since Rick Jamison did a Handloader article on the '89 I thought I'd post here as well.