Help with BC calculation

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Mysticplayer, Aug 16, 2004.

  1. Mysticplayer

    Mysticplayer Writers Guild

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    Just finished with my drop table of my 6.5 and 140gr Hornady SST bullets.

    Would appreciate if others could verify the BC based on the following data.

    Muzzle vel is 2975fps 10ft from muzzle.
    drop/come up at 800yds 15 min, 700yds 11min, 600yds 8min, 500yds 6min, 400yds 4min, 300yds 2.5min.

    Ranges verified with a Leica 800. Come ups based on Elite 4200 6X24 turrents. Come ups were repeatable. Amount per click has been verified at 100yds.

    All drops are based on a 100yd zero.

    From the program I am using, I am getting some very high BC's. Would love to see what your program says. Conditions are sea level, 85 F, 30% humidity.

    Appreciate the help...

    Jerry
     
  2. Dave King

    Dave King Well-Known Member

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    Something is ODD!

    Are you sure about the speed, 2975fps???


    The BC would be over .85 and I still can't get a trajectory match.
     

  3. danacobb

    danacobb Well-Known Member

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    I also tried running the numbers with a BC of 1.25 and thats still to low. I also tried running the numbers shooting at an angle. They came out closer with a 30-35 degree shooting angle, but only work from 300-600 or from 600-800 and there still not right on. Something's got to be wrong????

    Just ran the numbers again with a MV of 3350 and the numbers come out much closer. Also tried the match the trajectory using your come ups, and exbal calculates an MV of 3401 with using a BC of .512 but the 800 yd comes up shy at 13.5

    I'm just learning all the tools of this Exbal software, (got it last week). I'd have to say thats its a pretty good program.

    Probly didn't help, but I tried [​IMG] Good shootin

    [ 08-16-2004: Message edited by: Cobber ]
     
  4. milanuk

    milanuk Well-Known Member

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    Jerry,

    It would be helpful if you could tell us what your scope height above the bore is... using JBM on the web, it appears to make a significant difference. Me thinks your chrono may be b0rked? That's the only way I could come close to your numbers:

    Trajectory (Basic) Output

    --------------------------------------------------------------------------------

    Input Data
    Muzzle Velocity: 3250.0 ft/sec
    Ballistic Coefficient: 0.520
    Drag Function: G1
    Bullet Weight: 140 grains
    Sight Height: 2.50 inches
    Wind Cross Speed: 10 mph
    LOS Angle: 0 degrees
    Target Speed: 10 mph
    Zero Range: 100 yards
    Temperature: 85.0 °F
    Barometric Pressure: 29.50 in Hg
    Relative Humidity: 30.0 %
    Altitude: 0 feet
    Air Density: 93 % of Sea Level


    --------------------------------------------------------------------------------

    Calculated Table
    Elevation: 4.039 moa
    Azimuth: 0.000 moa

    Range Velocity Energy Momentum Drop Windage Lead Time
    (yards) (ft/sec) (ft-lbs) (lbs-sec) (moa) (moa) (moa) (sec)
    100 3063.7 2917.8 1.90 -0.0 0.5 16.0 0.095
    200 2884.8 2587.0 1.79 -0.6 1.0 16.5 0.196
    300 2712.9 2287.8 1.69 -2.1 1.5 17.0 0.303
    400 2547.6 2017.5 1.58 -4.0 2.0 17.5 0.417
    500 2388.8 1773.9 1.49 -6.2 2.6 18.1 0.539
    600 2236.0 1554.1 1.39 -8.6 3.2 18.7 0.669
    700 2088.1 1355.3 1.30 -11.2 3.9 19.4 0.808
    800 1946.1 1177.3 1.21 -14.2 4.6 20.1 0.957
     
  5. Guest

    Guest Guest

    Did you guys ever agree on the flight numbers? I'm going to need some help with my 6.5 264 WinMag. Can get you whatever numbers you need for factors. Thanks!
     
  6. Mysticplayer

    Mysticplayer Writers Guild

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    Jeff, I would have to say that my velocity is pretty much right on. Reason is simply this is not a new wildcat and there have been lots of shooters who have achieved this type of velocity. They have never achieved 3100 to 3200fps with a 24" barrel without blowing themselves to kingdom come.

    Pretty much impossible too with so little powder. In line with performance from its closest cousin, the 270 Win.

    Also, I have used this Chrony with other rifles and they have all "agreed".

    Now if the chrony cannot be trusted at 10ft. Why would I trust its readout at 100yds?

    So the only error possible is that the chrony is reading too fast. If that were the case, the BC would be even HIGHER. Now, that would be seriously screwy.

    I have attached a pic in another post (my new 6.5 toy) so that you can see that this is a very standard rifle and scope set up. Scope height is pretty much 1.5", etc.

    My suggestion is still that any 6.5 shooter get some of these SST's and shoot them. At 2975fps, do you get the same or similar drop?

    The correct BC may or may not be the 0.9 or 1 but it certainly is above 0.6. Either way, this is a bullet worth further investigation.

    Capable of shooting in the 0.1's too.

    Jerry
     
  7. Jeff In TX

    Jeff In TX Well-Known Member

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    Jerry,

    Sorry I'm not questioning your results only trying to understand them better. This stuff ain't rocket science, but there are the laws of physics and is usually pretty straight forward.

    I've never had much luck with my two chrony's. But getting an average velocity readings at exactly 100 yards and having your average muzzle velocity will establish your correct BC and then we can use the different drag models to better fit it to your results.

    Just trying to help. There are a lot of explanations for your results, but having exact numbers help.

    Jeff

    [ 08-18-2004: Message edited by: Jeff In TX ]
     
  8. Mysticplayer

    Mysticplayer Writers Guild

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    I told you something was weird. The vel was off a new Chrony at around 10ft. Did a few shots to get the vel. average. Have used this chrony to compare with other of my rifles and it matches. At worse, it is reading FASTER which would make the BC even higher!!!

    To match my data, I am getting BC at 1 and higher. Bizarre.

    Took it out yesterday morning and the come ups were again verfied. Then I started using the mil-dots at the preset mag (have confirmed mil dots subtend correctly at 100yds). Sure enough, bullets hit where they were supposed to.

    So even if the turrents moved more then the clicks indicate, the mil dots are using the same amount of MOA elevation for the distance as the turrents.

    Most of the shooting has been level to only a few degrees up. Weather has been warm to hot, dead calm and clear. Low humidity - perfect shooting weather.

    I have always known that the Hornady poly tipped bullets had higher real world BC's then what was printed. I also expected these 140gr SST to be much higher then the 142gr MK and Lapuas because the SST are much longer.

    Never thought it would be that high.

    I know the bullet is not going faster then 2975fps because I am only using an '06 case and 58.5gr of H4831SC. A 6.5Win Mag load uses a lot more powder to go over 3000fps. Barrel length is 24" so we are in standard/well known territory.

    Using Weaver bases and High Burris rings. haven't measured the scope height but not overly high.

    All you 6.5 shooters may want to give this bullet a look. It will stabilize in 9 twist barrels and it does shoot very well. Would love to hear if anyone gets similar results.

    Jerry

    [ 08-18-2004: Message edited by: Jerry Teo ]
     
  9. Jeff In TX

    Jeff In TX Well-Known Member

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    Jerry,

    Your results are very screwy and don't make much sense. But I learned long ago, never say never, cause screwy stuff is what this world go round.

    However, I would never trust a chrony. I have two as well as two Millenium CED chronographs. My chrony's have been back twice to be checked and recalibrated and they are so inconsistent it is not funny. But the two CED's are within a foot per second of each other when placed right behind each other. My two Chrony's can be 200 fps faster or slower than the CED's.

    If you want to figure out what you actual BC is. Move the chrony out to exactly 100 yards. Not close to a 100 or a laser ranged 100 yards, but exactly 100 yards and get an average velocity reading. Post your results and I'll let you know your actual BC is.

    Hope it helps,
     
  10. Mysticplayer

    Mysticplayer Writers Guild

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    Jeff, remember I was the first one to say the results were goofy. However, it certainly indicates the POSSIBILITY of a higher BC 6.5 140gr bullet.

    I hope others will try them and give some feedback. Should work in any 6.5-284 set up for the heavy 6.5 bullets. The SST's are longer then the 142MK and Lapua's so should have a higher BC.

    The actual value will need a bit more research.

    Let's see what other real world feedback we can get.

    Thanks,

    Jerry
     
  11. milanuk

    milanuk Well-Known Member

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    Jerry,

    Any chance of you posting this information over on either www.long-range.com or the 1k BR forum at www.benchrest.com? Some ballistic weenies hang around there and might be able to forward an idea as to what is going on. Try as I may, I can't bring myself to believe that a Hornady 140gr SST has a B.C. that much higher (0.2+!!) than any of the target VLD's on the market that aren't made out of some exotic material.

    Monte
     
  12. chessman

    chessman Well-Known Member

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    Use a G7 drag function and it comes out fine with a BC of ~.5.
     
  13. milanuk

    milanuk Well-Known Member

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    That's just it... it *doesn't* work out, because a G7 B.C. of ~0.5 translates to about a 0.8 or 0.9 G1 B.C. which seems entirely unrealistic. Yes a G5 or G7 B.C. is technically more accurate, but we are talking small amounts here. Figuring the drop for a 142gr SMK out of my 6.5-08 w/ a G1 B.C. of .595 as compared to a temperature/altitude/humidity corrected G5 B.C. of something like .409 (don't have it right in front of me so I may be off there) results in something like less than 1/2 MOA difference @ 600yds.

    Jerry,

    Another thing to maybe check would be to try a different box of the same lot of bullets, and a different box of a different lot of the SSTs. If the results are consistent, and we can't come up w/ some other explanation... man I hope Hornady is stocked up on these puppies, cuz once the LR competition shooters find out about this... [​IMG]

    Monte
     
  14. chessman

    chessman Well-Known Member

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    Ok, this is fun. I'm an aerospace engineer. I love ballistics. I can't explain everything, but I enjoy the study. First, a published BC should have been determined by testing over a variety of distances. What happens when we plug what we think we know into an equation, and the results don't match our prediction? We have long discussions about it on forums. Most of these posts focus on trying to prove that the measurement must be off rather than discussing whether or not the equation was correct. Some constant, variable, or group of variables must be incorrect if the results don't match the prediction.

    What makes one drag function more correct than another? If a certain standard drag equation defines a particular bullet shape, what happens if you build a different shape bullet? The answer is that the equation that defines the performance of one bullet doesn't fit all other bullets. BC is part of the equation. - just one variable. Change a constant, like the shape of the bullet, and the relationship of all the variables changes also.

    When a BC is published, do the manufacturers give you all the math behind their claims? What drag function best describes their bullet at a given velocity? Never have seen that on a box of bullets.

    The best way to predict bullet drop is to do exactly what Jerry is doing - measure the results and then try to find an equation that defines them. No single formula can fit all the variables. In this case, the G7 drag function more closely matches the measured results than the G1.

    F=MA