Wind Drift

[elktrout]

I am curious why the manuals and charts all base wind drift solely on ballistic coefficient. It seems like the weight of the bullet is not considered. Case in point: a 7mm bullet would typically have a higher BC than a .308 bullet, even though the bigger bullet was considerably heavier. I would think that the heavier object would be more difficult to move off its path than the lighter one.

Maybe this is an extreme comparison, but a golf ball would be harder to move off its flight path than a cardboard airplane shaped like a jet fighter. Granted the airplane would have a much sleeker profile (higher BC) than the golf ball, but common sense dictates that the golf ball would win the battle to stay on its flight path.


Do you have any actual shooting results that confirm or refute any of the wind drift theories?

 

[Silent Sam]

You have a lot of misconceptions. Weight or more correctly mass is a component of sectional density which is a factor in BC. Airplanes are not bullets. Bullets of different caliber (diameter) with the same form factor (shape), same construction, and same BC will fly essentially the same path in identical conditions. The larger caliber bullet will necessarily have to be heavier.

 

[helotaxi]

If you genuinely want to learn about this stuff, I highly recommend starting with the book Applied Ballistics for Long Range Shooting by Bryan Litz. He's a national level shooter and an aeronautical engineer; however the book is written for the non-engineer and is easy to follow, long on explanation and short on math (though the math and the formulas are there if you really want to get your geek on).

In your cardboard airplane/golf ball example, you'd likely find that the airplane had a very poor ballistic coefficient because it had a very low sectional density. Mass is factored into BC as mentioned above. But it is factored in the way that it matters which is the mass ratio to the frontal area (sectional density). If a projectile has a high mass, but also has a lot of frontal area, the additional drag from that frontal area will create the additional force needed to accelerate the projectile off its course. Wind drift is a drag function and that is why BC is the only physical property of the bullet that matters.

Sam-Just a point of clarification, if two bullets of different calibers had the same shape, IOW they were exact scale replicas of each other, and same construction, the larger bullet would actually have a higher BC because thanks to a higher sectional density. Volume, and thus mass, scales as the cube of dimensional changes. Take a 1cc cube that weighs 1 gram and has a sectional density of 1 (normalized, 1/1^2) and double its length and you get an 8cc cube that weighs 8g and has a sectional density of 2 (8/2^2)