300gn Berger Hybrid B.C. discrepancy.

[wadcutter]

The information that is supplied in Bryan Litz book for this .338 calibre bullet has an interesting discrepancy.
The book shows following G7 BC numbers.
3000 fps .419
2500 fps .418
2000 fps .413
1500 fps .420
Now compare this to the G1 values shown in the book and a different trend seems to talk place.
3000 fps .854
2500 fps .833
2000 fps .821
1500 fps .755
So the trend for G7 is for the B.C. value to deteriorate up until 2000 fps where it then makes an improvement in value.
Compare this to the G1 B.C. where values also deteriorate to 2000 fps but then <span style="color: #FF0000">take a sudden large drop in value</span>.
Now my field testing result tend to agree more with the G1 values out past 1000 yards where the velocity has dropped to below 2000 fps.
This also tends to agree better with the recent Snipers Hide BC testing.
Anyone else getting lower than published, BC numbers on this bullet below 2000 fps?

 

[BryanLitz]

Re: 300gn Berger Hybrid B.C. discrepancy.

The discrepancy stems from the differences between the G1 and G7 drag curves. The curves are shaped differently, which explains why the G1 and G7 BC's change differently with speed. Since the G7 drag curve rises more abruptly as the bullet slows in speed, it's actually not remarkable at all that the G7 BC goes up for a bullet that's compared to this standard.

A G1 BC of .821 and a G7 BC of .413 at 2000 fps represent the exact same performance at that speed. Likewise the G1 BC of .755 and the G7 BC of .420 also represent the same performance.

In other words, if you plug both these stepped BC's into a ballistics program and calculated a trajectory, they would be very close to identical. So I don't understand how you can see one being more accurate than the other since they're representing the same performance.

-Bryan

 

[wadcutter]

Re: 300gn Berger Hybrid B.C. discrepancy.

Bryan,
If you compare a projectile with a G1 BC of .821 against a projectile with a G1 BC of .755 both starting with a velocity of 2000 fps, the projectile with the BC of .821 will shoot flatter to 1000 yards. That's basic ballistics 101.

Now compare the same situation with the G7 BC numbers. The .420 G7 B.C. projectile will shoot flatter than the .413 G7 BC projectile. Again that is the basic as I understand it. So I can't see how you can say they are the same when the G7 BC numbers indicate an improvement in BC sub 2000 fps and the G1 BC numbers show a deteriation in BC sub 2000 fps.. I think that you may have something back to front.

Using "Shooter" with both sets of numbers starting out at the velocity of 2000 fps there is a 1.4 MOA difference between the two sets of numbers out to a grand. Now I am not saying that your numbers are wrong. I am just saying that they are hard to understand.

The fact that my sub 2000 fps field results also show some difference to your G7 B.C. numbers is anecdotal. This could be due to any number of things. I have theory on this that involves "critical rpm speeds" but I can't prove it.

 

[wadcutter]

Re: 300gn Berger Hybrid B.C. discrepancy.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bryan Litz</div><div class="ubbcode-body">
A G1 BC of .821 and a G7 BC of .413 at 2000 fps represent the exact same performance at that speed. Likewise the G1 BC of .755 and the G7 BC of .420 also represent the same performance.

In other words, if you plug both these stepped BC's into a ballistics program and calculated a trajectory, they would be very close to identical. So I don't understand how you can see one being more accurate than the other since they're representing the same performance.

-Bryan </div></div>
Bryan,
I plugged the numbers into "Shooter" a bit more and played about with a few things.
I got an interesting result when I used G7 number of .402 instead of .420, The numbers then seem to line up very very well with the G1 numbers of .755.
So is it possible that you spoonerised the .420 number for .402, or it may be a publishing error?

 

[BryanLitz]

Re: 300gn Berger Hybrid B.C. discrepancy.

Wadcutter,

I've double checked the numbers, and they're correct. I think we're mis-communicating about these BC's.
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">If you compare a projectile with a G1 BC of .821 against a projectile with a G1 BC of .755 both starting with a velocity of 2000 fps, the projectile with the BC of .821 will shoot flatter to 1000 yards. That's basic ballistics 101.</div></div>

I agree, but that's not what I'm saying. The bullet has a G1 BC of .755 <span style="font-weight: bold">only at 1500 fps</span>. It has a G1 BC of .821 <span style="font-weight: bold">only at 2000 fps</span>. So in order to make use of this information in a ballistics program, you would have to enter the BC's by velocity bands. You can't compare the BC of a bullet from one band to another.

Looking at the plot of drag curves and data points on page 547, you can see the drag of the G7 projectile (the dark black line) is much higher than the G1 line (gray) below Mach 1.5. The actual drag of this bullet is in between those two, but closer to the G7 line. So as the bullet slows below Mach 1.5, the curves diverge. The drag of this bullet is greater than the G1 curve, so the BC goes down in relation to the G1 standard. However, the drag is lower than the G7 (black) line, so the BC goes up in relation to this standard.

There is nothing wrong with a G1 BC going down while a G7 BC goes up, that's entirely possible and correct in this case.

When I enter the step-wise defined BC's into shooter for this bullet, and calculate trajectories out to 1500 yards, I get 45.6 MOA for the G1 BC's vs 45.9 MOA for the G7 BC's, which is the minor difference I would expect.

I think I may know what you might be doing wrong. Some people think that if their bullet slows to 2000 fps (for example) that they would use the 2000 fps BC for the bullets whole flight. Likewise if the bullet slows to 1500 fps, they would use that BC for the bullets whole flight. This is not the case. You have to apply the high velocity BC's to that portion of the bullets flight, and as the bullet slows down, apply the appropriate BC for each velocity step. So in other words, the 1500 fps BC's only apply when the bullet slows to that speed, and that's it.

When you do it this way, you get remarkably close predictions for step-wise defined G1 BC's and step-wise defined G7 BC's, with the G7 being slightly more accurate. If you have to use only 1 single BC input, the average G7 will provide a much better prediction than an average G1 because of there is much less variation in the G7.

-Bryan

 

[RemingtonSPS]

Re: 300gn Berger Hybrid B.C. discrepancy.

If you only enter one BC into shooter it will use that BC as an average over the entire velocity range.

Bryan is talking about stepped BC's where you enter the BC for each step in velocity i.e. 3,000fps, 2,500fps, 2,000fps, 1,500fps with a BC for each velocity. This method is more accurate than using an average BC for the entire velocity range. This method is the one i use and its extremely accurate. If you have Bryans book it will give you the average BC to use or the BC's for each velocity range if you want to step them. If you plug in the BC's for each velocity as he has it in the book you will be really close.

 

[wadcutter]

Re: 300gn Berger Hybrid B.C. discrepancy.

Hi Bryan,
Thanks for your detailed explanation which I accept.
Thank you also for taking the time to answer my questions.

In my testing I have used both your average G7 BC value and also your banded G7 values. I have got very good agreememt with my trajectory using these values out to 1000 yards. So I am quite happy with your G7 BC value in this range. I did not simply use the lower value.

I have shot extensively over the last few months, at distances in the 1000 to 1800 yard range and also out to and beyond 2300 yards.

For distances where the velocity has dropped below 2000 fps there is a steady divergence between my trajectory and that predicted by my ballistic calculator "Shooter".

As my results are from the agricultural method of "shoot and see what you get", my results are anecdotal only. Still, I have to use different BC numbers where part of the trajectory will be below 2000 fps or I will miss the target. So I will have to stick with what my field results give when using my particular shooting equipment.

It is interesting that I do not see this divergence when testing the Sierra 300gn SMK out to near transonic and your G7 BC values for that bullet. It is only with the Berger hybrid where this seems to occure.

Such is the reason why we validate our trajectory to our equipment..

 

[wadcutter]

Re: 300gn Berger Hybrid B.C. discrepancy.

I have been mulling this over a bit as this concept has increased my understanding of how the G7 and G1 BC drag curves compare against each other.

Quite frankly this has made my head spin a bit. So to clarify what you are saying Bryan.

At 2000 fps a G7 BC of .413 is equivalent to a G1 BC of .821.
and........
At 1500 fps a G7 BC of .420 is equivalent to a G1 BC of .755.

Also.

I have looked at the plot on page 547 as you sugest and can see the two drag curves. The last four points that are plotted on the chart in about the mach 1.7 zone appear to be below both the G7 and G1 drag curve lines. What does that mean?

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Looking at the plot of drag curves and data points on page 547, you can see the drag of the G7 projectile (the dark black line) is much higher than the G1 line (gray) below Mach 1.5. The actual drag of this bullet is in between those two, but closer to the G7 line. So as the bullet slows below Mach 1.5, the curves diverge. The drag of this bullet is greater than the G1 curve, so the BC goes down in relation to the G1 standard. However, the drag is lower than the G7 (black) line, so the BC goes up in relation to this standard.
</div></div>

 

[BryanLitz]

Re: 300gn Berger Hybrid B.C. discrepancy.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">At 2000 fps a G7 BC of .413 is equivalent to a G1 BC of .821.
and........
At 1500 fps a G7 BC of .420 is equivalent to a G1 BC of .755.</div></div>

100% correct. The G1 and G7 curves are different, so the G1 and G7 BC's for a bullet change differently with velocity.

Regarding the data points, there is some discussion starting on page 272 which addressess the data points and scatter. There are several possible reasons for the 4 low drag points around Mach 1.7 to be slightly below both curves; the most likely explanation is experimental uncertainty/error. Those 4 points were probably measured with sensors between 600 and 1000 yards where there is slightly more uncertainty in the bullets path compared to 0-600 yards. So if the bullet passed 12.5 feet from the sensor at 1000 yards instead of the documented 11.5 feet (for example), that could be a source of minor error.

The drag points being low would suggest higher BC at lower speed though, not the lower BC at slow speed that you're seeing in your shooting.

You are not the first to observe the drop-off in BC for this bullet below 2000 fps; others have told me this too. However, at least as many people have told me the average G7 BC tracks perfectly all the way down to transonic. I haven't done extensive ELR testing on this bullet myself to focus on velocities below 2000 fps, but I plan to. I suspect the various observations may be related to altitude effects on the bullets stability. However 2000 fps seems high to begin seeing transonic stability effects.

Thanks for bringing this up, I think there are others who will benefit from this discussion.

-Bryan

 

[wadcutter]

Re: 300gn Berger Hybrid B.C. discrepancy.

Bryan,
Thank you for taking the time to address my questions. You have made me aware of things relating to drag curves that I failed to comprehend before.
They certainly work slightly different to how I thought they did.
When you do get arounf to doing some ELR testing I would be very interested to see your results.
Cheers.

 

[Hoverp]

Re: 300gn Berger Hybrid B.C. discrepancy.

Same here! Great discussion and learning going on! Love the book also Brian.