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Question on bergers bc

 
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  #1  
Old 02-15-2009, 10:33 PM
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Question on bergers bc

Just wondering when berger lowered the bc on their bullets. I was on their site and they state a 140gr 6.5 has a bc of .595. I havent bought any in awhile and all the boxes I have say .640 What made them lower the bc, through testing did they decide it wasnt as high as they have stated in the past.

I shoot alot of 130 normas .264, norma says they have a .540 bc but through shooting them its been closer to .60 to make the trajectory work out. Just wondering how different companys come up with there bcs and how legitimate most are.
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  #2  
Old 02-15-2009, 11:05 PM
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Re: Question on bergers bc

Sandman,
There have been many who have wondered the same thing about the recent change of our BC's. To provide the best answer, we prepared a detailed explanation that's available on our website, the link to the article is here:
http://02b0516.netsolhost.com/blog1/?p=12
The article specifically answers your question and then some. If you have any questions after reading the above I'll be happy to discuss it with you.
-Bryan
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And: Accuracy and Precision for Long Range Shooting

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  #3  
Old 02-16-2009, 03:30 PM
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Re: Question on bergers bc

What I'm missing is how anyone should apply an 'average BC' within their ballistic software.
Isn't it the software's job to do this?
We enter BC, local conditions, and muzzle velocity. The software should take it from there using retard coefficients(in one form or another), or drag tables with velocity and BC iterations the whole way(like JBM's).

What if a 6.5WSM is launching a 140VLD at 3500fps(beyond your average velocity)?

The trick I suppose, is entering a correct BC for the muzzle velocity.
BCs aren't usually listed as based on XXXXVelocity. Heck we aren't even given based conditions(StdMetro, StdArmy, ICAO, etc). It's good that you take it to ICAO, and base it on a velocity. Is it stamped as such on the box?
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  #4  
Old 02-16-2009, 04:26 PM
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Re: Question on bergers bc

Mike,
The way to solve all of the velocity complications is to use G7 BC's. With G7 BC's, there is practically no variation with velocity and you can use one BC without consideration to what velocity it's valid for.

Quote:
What if a 6.5WSM is launching a 140VLD at 3500fps(beyond your average velocity)?
Shortly beyond 200 yards that bullet will fall below 3000 fps, and be in the window that the average BC is calculated for. The bullet will spend most of it's long range flight between 3000 and 1500 fps, which is why that window was chosen.
Having a common velocity window also allows 'apples-to-apples' comparisons of the BC's between different bullets. If each bullet's BC were advertised for different speeds, you wouldn't know which is really better. This is why our approach of making BC's based on an average velocity (that's representative of long range flight) makes us compare less favorably to some of the competition. If others base their BC's on high velocity only, it will yield a higher BC, but that BC will not be representative of the bullets actual performance over long range.

The fact that you need to be aware of and account for velocity with BC is a problem with the G1 standard that we've always used to reference BC's to. There are all kinds of ways to deal with the velocity dependence of BC's like defining it in multiple segments, or giving an average value but the best way is to use a better standard. You shouldn't have to worry about velocity dependence, and the G7 allows you to ignore it and generate more accurate trajectories, and make more meaningful comparisons between different bullets.

JBM works how a ballistics program should work, the problem is that we give it G1 BC's, and that's not really the right way to use it for long range bullets.
The classic G1 BC is referenced to the G1 standard projectile which is a blunt nosed, flat based bullet. When you enter a G1 BC into a ballistics program, the program applies the drag curve for the G1 projectile, which is much different than the drag curve for a typical modern long range bullet. That is why the BC changes with velocity: because the modern pointy boat-tail bullet compares differently to the G1 standard projectile at different velocities.

When you tell the program you're giving it a G7 BC, it uses the drag curve for a pointy boat-tail bullet which is the same shape as our bullets. Since the drag curve of the G7 standard projectile is the same as our bullets, the G7 BC is constant for all speeds.

When I say 'drag curve', I'm referring to how the bullets drag coefficient changes with velocity. This is similar to the 'retard' coefficient often referred to. The shape of a bullets drag curve depends on the shape of the actual bullet. Bullets of similar external shape will have similar drag curves, and will have drag coefficients that compare equally at all speeds.

What you cannot do is enter a G1 BC and tell the program it's a G7 BC. Doing so will produce meaningless results. You have to give it a G7 BC which is numerically lower than a G1 BC.

I'm working on a resource that will bring the transition to G7 BC's closer to reality. In the mean time, I can provide G7 BC's for any Berger bullets on request. You can use them to calculate trajectories in JBM, or any other software that accepts BC's referenced to the G7 standard.

Advertised BC's (G1 or G7) need to be corrected for standard atmospheric conditions or they're useless. Ballistics programs apply the corrections based on how different your conditions are from standard conditions. I believe all companies correct their BC's to standard conditions, it should go without saying.

Take care,
-Bryan
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And: Accuracy and Precision for Long Range Shooting

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  #5  
Old 02-16-2009, 05:14 PM
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Re: Question on bergers bc

Hi Sandman,

Just to make sure that you're thoroughly gang-answered by the Berger crew here, let me add a bit to Bryan's comments. Sierra uses multiple BC, velocity "break-points" in an attempt to "force fit" what the bullet does in the real world to what the G1 model will actually accomdate. The bottom line here is that when you use a drag model to work trajectories for a bullet that differs greatly from the standard for that model, problems arise. Since the G1 is the single drag model that is used for virtually all US commercial markets (to reduce confusion for shooters trying to compare, say, a FN design to a VLD design, where the flat nose could concieveably show a much higher BC; different drag models for each), Sierra was just trying to stay consistent with the industry norm, and make the best of a bad situation. The best solution, as Bryan described, is to use the appropriate drag model for the bullet you're working with.

Just another pebble for the pile,

Kevin Thomas
Berger Bullets
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  #6  
Old 02-16-2009, 09:17 PM
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Re: Question on bergers bc

So.....
How should we apply an 'average BC' within our ballistic software?

The software assumes entered BC is correct for launch velocity entered, right?
It doesn't know the entered G1 BC is only .595 -once velocity has dropped to 2500fps.
It doesn't know that the .595 entered is .644 as launched at 3500fps. I have to enter .644 for BC at that MV.
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  #7  
Old 02-16-2009, 09:58 PM
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Join Date: Feb 2009
Posts: 15
Re: Question on bergers bc

Thank you guys for the quick answers it is very informitive. I was wondering what the g7 bc of the 264 130 and 140s would be for berger.


So far out of my 6.5-06ai im getting 3325 with the 130 normas and to match my data I shot in the field 100yd zero 400yards 4moa and 700 yards 10 moa I had to raise from normas quoted bc of .54 to .60 because this is a g1 bc based off a slower velocity then what I shot them at correct. With a g7 bc there shouldnt be that much difference between qouted and actual no matter the velocity with an adjust atmosphere is what im assuming.

I am making plans to build another 6.5-284 and was wondering what your opinion would be. To shoot 130s and get more velocity or the 140 and have a little higher bc.

Thanks for any respones its much appreciated.
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