Balistic Coefficient
- Thread starter Easy E
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Very conflicting. The methods we have at our disposal to calculate BC 100% would agree... While some maintain the belief that BC does not change based on velocity because BC is determined by physical properties of the projectile itself, based on sectional density and form factor, and what does change is its cD, and that is what you're measuring.
Frankly, one could go either way with their beliefs, and give a constant cD with a varying BC using one set of definitions, while others posture a variable cD and a static BC.
I had to build my own ballistic solver to really understand cD and BC and how each work within current ballistic models.
I think the two terms are easily misunderstood and it doesn't help that the current method to measure BC doesn't align with the above static BC model. We would need a new formula to translate velocity difference between 2 measured points into their cD values and then another formula to determine the BC from that cD value within the given speed....at least imo.
-Matt
Frankly, one could go either way with their beliefs, and give a constant cD with a varying BC using one set of definitions, while others posture a variable cD and a static BC.
I had to build my own ballistic solver to really understand cD and BC and how each work within current ballistic models.
I think the two terms are easily misunderstood and it doesn't help that the current method to measure BC doesn't align with the above static BC model. We would need a new formula to translate velocity difference between 2 measured points into their cD values and then another formula to determine the BC from that cD value within the given speed....at least imo.
-Matt
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Thanks for the replies! Matt great respect, and a honor to have you reply! I have watched many of your videos about BC. NSA list a BC for its various 457 ammo. I ordered the new ZAN 380g to try and they don’t list a BC! So I guess it’s safe to say, I need to do my on Calculations, even on the NSA slugs to get it accurate! I have the new Atn xsight 5lrf smart scope, and I wanted to set it up for November deer hunt!
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I'm Matt Stubbers not Matt Dubbers, lol.
Here is a good article to read.
Drag Coefficient and Ballistics Coefficient Explained - Hard Air Magazine
I have many times referred to the terms Drag Coefficient and Ballistics Coefficient. Here is an attempt to explain how they are related to each other.
hardairmagazine.com -Matt
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If you want to use the BC for calculations for slugs, do not use the G1 reference law at high speeds, as it will give artificially high numbers. You would be better using RA4 or SLG0 if you have software with it available.
The reference drag law you want needs to be the one where the drag law shape most closely matches the drag law shape of your projectile, then any changes in speed should only need a small change in BC to match.
The reference drag law you want needs to be the one where the drag law shape most closely matches the drag law shape of your projectile, then any changes in speed should only need a small change in BC to match.
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I haven’t got into the ATN software yet so I’m not sure what it calls for yet like with Strelock I used the numbers provided by the software
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For a bullet using the RA4 drag model, BC stays consistent for most all practical velocities.
With GA pellets and some G1 slugs, when you get outside the typical speed range, measured BC might vary significantly.
Some will say that the pellet slows down faster at high speeds, so it will have a lower BC. True, it slows down faster at high speeds (more drag), but the measured BC can actually be higher at high speeds. That sounds great, but not really that great. A modern round nose pellet does better than GA expects it to at high velocity, so the measured BC becomes exaggerated for that short distance after leaving the muzzle.
If you’ve got some of the newer, round nose, high BC pellets that are being shot at over 950fps, they behave almost like a bullet. You could probably use the RA4 drag model, and get a more consistent BC. The RA4 BC will be lower than GA, but that doesn’t matter. Consistency across a wide velocity range is what you want.
It’s possible to use a multi-BC function to map a high velocity GA or G1 trajectory. It works, but it’s a band-aid.
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Hopefully not to far off topic, but what are your opinions on, calculating sight height with a canted base, say 30moa. Where do you measure from. Objective including the sun shade or at the turrets.
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Yes - off topic. I take the scope height measurement at the erector tube gimbal. Distance to target is also measured from that point. That point is approximately at the magnification ring.
The gimbal is the vertex of the trajectory angle. It is the point from which angular compensation is calculated.
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