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Thread: What is a 'Good' Ballistic Coefficient?

  1. #1
    Sharp Shooter

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    Default What is a 'Good' Ballistic Coefficient?

    What is the magic number? If I'm looking at the IMDB I disregard any movie scoring less than 6.2 as it's most likely, rubbish. Is there a significant figure for air rifle pellets above or below which they are probably not worth the expense?
    All contributions are gratefully accepted. Ta!
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  2. #2
    Protea Benchrest (Air) Team '15/'17

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    I do not think you should hold your breath for a definitive answer.

    I think the 0.177 JSB Heavies and 0.177 JSB Monster pellets have the best ballistics for power level up to 20fpe.

    There is no magic number and there is no significant above or below figure that have any expense value - my view.
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  3. #3
    Sharp Shooter

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    Thanks Juniork. I always assumed that there must be a point on the BC scale where the pellet just doesn't perform adequately. It's all down to 'what works for me' I guess.
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  4. #4
    Marksman

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    The way I see it is that BC changes depending on the velocity you shoot the projectile at. So you have to calculate your own BC for the setup you use as it will differ from the "factory" values. A longer projectile with more weight will give a higher BC but it might not be launched accurately if your barrel twist rate is too low and the speed for that twist is too low as it can't stabalize the projectile. So a high speed with a long heavy projectile and high enough twist rate will yield best BC. Then the drag created by the projectile design plays a role. Not too sure how it works with pellets but that is why a boat tail taper will give better terminal preformamce than a typical flat base in centre fire cartridges. I am not sure about all the different variables used in the calculation of the pellet BC model vs the G1 or G7 models. You can read about it here Ballistic coefficient: What is it? Part 2 | Air gun blog - Pyramyd Air Report

    This is why i want to try a JSB Beast in .177 at a high velocity to see how it will compare to a .22 rifle with almost the same weight. Longer projectile with smaller cross section should = better BC at high velocity. That is if the barrel twist can keep up. Hope it works well in the s510 ultimate sporter that is due to arrive soon.
    Last edited by HugoB; 12-09-17 at 23:56.
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  5. #5
    Sharp Shooter

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    Sorry that I've been slow to respond. Thanks for the clarification. I won't ake too much notice of BC in future if it's dependent on power etc. I'll just experiment with ammo as I always have. I'd hoped there was 'a clue in the title'.
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  6. #6
    Sharp Shooter

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    BC even differ from lot to lot of the same pellet. Because BC varies with velocity over the trajectory, it's a dynamic parameter. Advertised BC values are only a snapshot of that projectile's aerodynamic efficiency at a particular set of conditions. Without knowing these conditions, BC comparisons are not worth much.
    Two methods to determine the average BC for a projectile:
    1. Decrease in velocity over a known distance. The most reliable method, but the least practical. For this you need two calibrated chronies at the two distances.
    2. POI drop over a known distance, with measured muzzle velocity. Using software (Chairgun), this test is easier.

    Using method 2, I have determined BC's for some lots of .177 JSB pellets in 8.44 gr and 10.34 gr. All measurements were different from the advertised values (lower). My measured BC values produced more accurate trajectory plots in Chairgun than using the default values.


    My 2c added to the grey picture of BC's.
    Last edited by kettieklip; 20-09-17 at 08:06.
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  7. #7
    Sharp Shooter

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    I think you're not asking the right question - maybe?

    a pellet has the aerodynamics of a beer can and flies just as far.
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