WIND DRIFT VS. B C QUESTION

[dgeesaman]

BC is supposed to be a true coefficient of drag, meaning that all the physical differences between two bullets are already “baked in” to the number, for drift and drop purposes.

The theory would be that if a 6mm and 30 cal had exactly the same BC, then if they were shot at identical velocities, the center of each impact would be in exactly the same place, at every distance and condition.

This is why our trajectory calculators like JBM Ballistics do not actually require that we put in the correct caliber and bullet weight to produce the drift and drop chart. It relies solely on the inputted BC for drift and drop.

This is 100% correct to the math.

The people who write the ballistic models will tell you that any error comes from inaccuracy in the ballistic coefficient model, inaccurate wind call, and a small about for variance between individual bullets.

 

[davidjoe]

I use to believe that BC numbers says it all but If you shoot enough 6mm bullets vs big 308 bullets at a 1000y you do start to see something in the wind that the calculators missed when it comes to big heavy bullets

Rifling and spin rate, because they are intrinsic to our specific guns as opposed to the projectile, might create effects that escape manufacturers’ assigned bullet BC.

Rifling grooves cut into bullets that are spinning as they travel through the air must generate turbulence. The grooves cannot be perfectly accounted for in BC calculators because the depth, surface area, shape, pitch and number of them are unique to a gun. A defect from wear or otherwise further can’t be accounted for.

I’ve often thought that a faster spinning “over stabilized” bullet increasingly resists pointing into the total wind vector it sees, which changing vector induces pivots over long paths, for the same reason that a faster spinning gyroscope resists changing orientation, and because they “fly” with a slight point up attitude relative to ideally stabilized bullets.

If true, both of these would affect aerodynamic efficiency, probably “net” negatively, and both would escape standard BC calculators by reason of being gun dependent variables.