Question on bergers bc

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by sandman264, Feb 15, 2009.

  1. sandman264

    sandman264 Member

    Messages:
    16
    Joined:
    Feb 4, 2009
    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.
     
  2. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

    Messages:
    633
    Joined:
    Mar 8, 2007
    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
     

  3. Mikecr

    Mikecr Well-Known Member

    Messages:
    4,264
    Joined:
    Aug 10, 2003
    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?
     
  4. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

    Messages:
    633
    Joined:
    Mar 8, 2007
    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.

    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
     
  5. Kevin Thomas

    Kevin Thomas Well-Known Member

    Messages:
    1,256
    Joined:
    Feb 16, 2009
    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
     
  6. Mikecr

    Mikecr Well-Known Member

    Messages:
    4,264
    Joined:
    Aug 10, 2003
    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.
     
  7. sandman264

    sandman264 Member

    Messages:
    16
    Joined:
    Feb 4, 2009
    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.
     
  8. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

    Messages:
    633
    Joined:
    Mar 8, 2007
    Mike,
    You can just enter the average BC into the program.
    If you enter a high BC that's only valid for the muzzle velocity, the program doesn't know to erode the BC as the bullet slows down. If you enter .644 at the muzzle velocity of 3300 fps, the program will apply .644 for the entire trajectory, which will be too high for most of the flight.
    However, if you enter the average BC, it will be low for the first part of the trajectory when the bullet is fast, it will be very close to right on for much of the flight, and it will be a little high at the end of the flight. The net effect is that it will generate a trajectory that's much closer to reality than a BC that's only valid for muzzle velocity.

    Sandman,
    The G7 BC's for the 6.5mm 130 and 140 VLD's are: 0.282 and 0.304. These are valid for all flight speeds.
    As to your question about the 6.5-284, go with the heavier bullets. Heavier bullets of similar shape will always be better performers at long range even though they suffer a depressed muzzle velocity. By the time they get to 1000 yards, they're faster than the lighter bullets that started out faster and lost velocity quickly. About the only count that light bullets beat heavy bullets is the flatness of the trajectory. This is only an issue if you don't have a rangefinder, and at long range, the heavy bullets end up flatter than the light ones anyway.

    A word of caution about deriving BC from drop data...
    If you're determining your drop at a given range by how much scope correction is needed at that range, you have to make sure your scope is calibrated, and you know how much each click is worth. In other words, many scopes that advertise 1/4 MOA clicks can actually move the crosshairs more or less than 1/4 MOA per click. If you think you've moved your crosshairs some amount at 700 yards, but they actually moved more or less than you think, your calculation of drop and BC will be inaccurate.

    -Bryan
     
  9. haywire05

    haywire05 Well-Known Member

    Messages:
    80
    Joined:
    Dec 25, 2008
    This is for the berger boys I have bought thousands of dollars of your guys ammo . Do you guys not have a book out yet or a website where we can get the reloading info for your ammo .

    It is a pain in the butt phoning all the time especially with my job as you are usually closed when I can phone . Cheers Jason
     
  10. grit

    grit Well-Known Member

    Messages:
    1,375
    Joined:
    Mar 23, 2005
    Bryan and Kevin,

    Thanks a bunch for being on here to answer our questions!
    I would appreciate the g7 for the 30 cal 210.
    Also, are these published somewhere? How do we get them for various bullets (yours and others)?

    I thought exbal let you choose between g1 & g7. Is this not so??
     
    Last edited: Feb 18, 2009
  11. Kevin Thomas

    Kevin Thomas Well-Known Member

    Messages:
    1,256
    Joined:
    Feb 16, 2009
    Jason,
    I'm still getting the kinks worked out with my being added to the Berger email (again, the new kid) but you can email those requests in, instead of calling. There should be something in the works that will make those phone calls easier, too.

    Once my email is up and running to speed, I should be able to handle the data requests and will be happy to do so.

    Kevin Thomas
    Berger Bullets
     
  12. Kevin Thomas

    Kevin Thomas Well-Known Member

    Messages:
    1,256
    Joined:
    Feb 16, 2009
    Grit,
    I'm going to defer to Bryan here, since he's the one dealing with the ballistics software, and is better qualified to answer this than I.

    As far as getting the G7 BCs from other manufacturers, I doubt it. Virtually all US makers utilize the G1 model, simply to avoid confusion and allow a more direct comparison between the products. As shooters become more knowledgeable concerning BC, the makers will have a greater demand for BCs based on appropriate models that are better suited to the bullet in question, be it G1 through G7 or GL. With some of the computer programs out now, we're getting to that point, but as I said, Bryan's better qualified to answer this than I am.

    Kevin Thomas
    Berger Bullets
     
  13. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

    Messages:
    633
    Joined:
    Mar 8, 2007
    Grit,
    The G7 BC for the 30 cal 210 grain VLD is 0.315.

    I don't own exbal, so I don't know if it can accept G7 BC's or not.
    Here's a link to a very good (free online) ballistics program that lets you use G7 BC's:
    http://www.eskimo.com/~jbm/calculations/traj/traj.html

    Just enter your conditions and be sure to select G7 if you input the 0.315 BC for the 210 VLD. If you don't tell the program you're giving it a G7 BC it will assume you're giving it the 'ol G1 and it will produce useless results. The output of this program is quite accurate if it's given good input.

    There is currently no resource available to get G7 BC's from all brands. As Kevin said, it will be an uphill fight to get most of the other bullet makers to convert. We're working on materials that are designed to bring the G7 referenced BC's into the mainstream. In the mean time, I'm happy to continue providing G7 BC's for Berger bullets on request.

    -Bryan
     
  14. grit

    grit Well-Known Member

    Messages:
    1,375
    Joined:
    Mar 23, 2005
    Thanks.

    I don't want to take advantage of your time. But I have a few more questions.

    I ran the numbers for the 6.5 140 at 2600 fps using the g1 and g7 models. The numbers to 1k were virtually identical.

    Can you illustrate the differences in using the two models. I will get specific. I am shooting the 210's at 3013 fps. Max range 1500 ish.
    Alt, 5000'
    pressure at altitude, 25.4"
    temp, 50
    humidity, 24%

    Next question, I don't see an option to use the g7 on my Nightforce xbal software. I thought it had it.
    However, The program offers the option to define multiple bc's at certain velocities.

    For example, if I choose the 308 175 Smk the program shows two columns. Looks like this:

    BC - Lower velocity limits
    .505- 2800 fps
    .496 - 1800 fps
    .485 - 0 fps


    Now, when I choose your 210 it looks like this:

    BC - Lower limit
    .616 - 0 fps


    Perhaps you could provide me with bc's at lower limits like above to better utilize the program I have??

    If the G7 model or others are more accurate, I am all for switching. In the mean time, I'd like to use what I have as accurately as possible.

    Thanks again for your time!