Results 1 to 4 of 4

Thread: The truth about ballistic coefficients.

  1. #1

    The truth about ballistic coefficients.

    While wombling around the internet i came across this.

    Are ammo manufactures fabricating results for sales?

    worth a look,

    ATB Scooby

  2. #2
    Quote Originally Posted by whatwouldscoobydoo View Post
    While wombling around the internet i came across this.

    Are ammo manufactures fabricating results for sales?

    worth a look,

    ATB Scooby
    Very interesting.. My only observation (and it is hinted at right at the end... after they have pulled the manufacturers apart! lol) is that, BC is very dependant on velocity... Ballistic coeficients are applied in velocity bands or ranges and then averaged out to arrive at the one BC figure... If the muzzle velocity is different in the manufacturers test and the authors test then the overall BC will be different...

    The ONLY way they would be able to produce accurate numbers to support their claims is if they used the same barrel with the same twist, the same length... same powder, primer and case, not to mention seating depth etc to produce the same muzzle velocity... (what happens inside the barrel will have an effect on what happens outside the barrel, even if the final MVs are the same, hence the reason one can get significant variations in group size even with the same MV by changing powder/primer/cartridge dimentions etc!) Without that, the data is meaningless...
    Last edited by Vipa; 09-06-2015 at 13:13.

  3. #3
    Yes, G1 BCs are very velocity dependent with a 15% difference across a 3,000 fps MV down to say 1,500 fps some few hundreds of yards away not at all uncommon. For short range shooting out to 250-300 yards, G1 isn't a problem because the speed loss is relatively small. It is however useless for the long-range shooter. Manufacturers rarely lie, but they may quote a quite legitimate G1 BC, but which only applies at speeds above 3,000 fps for example.

    G7 BCs are much more accurate and consistent for long-range boattail designs and will usually see a 3% or so variation in actual BC in a 500-1,000 yard flight. They must be run within a G7 program, but all decent software: Berger's, JBM, QuickLOAD etc offer this facility. Not all manufacturers quote G7s yet, but most are available thanks to independent testing by Bryan Litz. All Berger and Lapua L-R bullets are quoted in both formats. For these two companies at any rate, their quoted G7s are very accurate indeed assuming the enquirer understands the relevance of and importance of inputting other key values before producing a range card - temperature, altitude and actual scope height above the boreline rather than just taking the defaults as supplied.

    The other factor in this is twist rate and stability. Recent research has shown that a marginally stabilised bullet that appears to function perfectly to the user will often lose a significant chunk of real BC performance through not being spun fast enough. To take an example, the 100gn Hornady BTSP is 1.083" long. Assume that a short barrel 243 Win rifle gives it 2,700 fps MV. (Less? 2,500-2,600) and we're talking a 1-10" twist barrel.

    Litz gives this bullet an average BC (3,000 - 1,500 fps) of 0.385 G1, but at the lower velocities we're looking at here, that'll drop to below 0.380; G7 average 0.198 which you can pretty well take to the bank irrespective of velocity and over all normal distances. However, that assumes full stabilisation.

    Run the bullet through the Berger Ballistics twist rate calculator (uses the Miller formulae) and you get with a 10" twist rate.

    2,700 fps .......... 1.31 Sg ................ BC compromised by 6%

    2,600 fps .......... 1.30 Sg ................ BC compromised by 6%

    2,500 fps .......... 1.28 Sg ................ BC compromised by 7%

    Those are in standard ballistic conditions (59-deg F / 29.92 inches mercury pressure) Going up high mountains (lower atmospheric pressures) improves things as does shooting in warmer air. Shooting in a very cold winter snap in anticyclonic conditions makes the loss of BC significantly higher for the bullet in marginal stability and these factors degrade a bullet's L-R performance in any event.

  4. #4
    Nothing new there, BC's have always had a sales value and some companies have historically over egged the BC to beat the competition.
    Only way to really know is test em yerself.
    A young man who isn't a socialist hasn't got a heart; an old man who is a socialist hasn't got a head.
    I have summat for sale; here's the M̶i̶d̶w̶a̶y̶ Brownells UK price... effin jokers.
    "The .30-'06 is unstable at close range" - Ahahahahhahh!

Similar Threads

  1. The Truth
    By Yoda in forum Off Topic
    Replies: 45
    Last Post: 25-12-2014, 02:59
  2. Never give up, the truth will always prevail
    By buck52 in forum Deer Stalking General
    Replies: 13
    Last Post: 16-11-2011, 09:57
  3. C,mon tell the truth
    By Brough in forum Deer Stalking General
    Replies: 11
    Last Post: 01-02-2009, 23:58
    By sikamalc in forum Deer Stalking General
    Replies: 22
    Last Post: 09-07-2008, 17:37
  5. Too close to the truth
    By Rob Mac in forum Videos
    Replies: 2
    Last Post: 06-11-2007, 17:48

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts