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BALLISTIC COEFFICIENT AND WIND DRIFT

The trajectory and wind drift of a bullet is determined by the speed and BC of a bullet.  So, if any one of these differ by even a small amount, the trajectory and wind drift numbers will differ as well.  For two bullets to go over the same trajectory with the same wind drift, over a distance, it is required that the combination of speed and BC be the same.  The bullets must be the same shape and fired at the same speeds.  To get two bullets of differing construction or shape to do this, is usually a matter of luck and happy circumstance. One can see from the illustrations below that speed has more importance for trajectory while BC is of greater importance for resistance to wind drift.

Where two different bullets are used that have different speeds and BC numbers, it becomes so complicated that it is almost not worth trying to get them to go to the same point of impact, let alone fly over the same trajectory.  If one bullet is slower than the other, but also has a higher BC, one could fiddle around with the speeds and try to get close. However much you try, the trajectory and wind drift will not coincide.  See the charts below. 

Where one bullet is slower, as well as having a worse BC, there is no way that the two will go over the same path or have the same wind drift to 200 meters or yards. If two differently shaped bullets have the same point of impact, the impact point will usually be at one given distance and, anywhere else on the trajectory the impact point will be different, because the trajectories differ. See the first illustration below.

The usual dilemma is that one requires a similar point of impact for a soft and a solid.  The problem is of course that a soft and a solid should be different shapes.  Where a soft and a solid are the same shape and weight, neither will work optimally.  If a solid has a blunt, round nose with a flat base, a soft of the same shape will be all but useless over distances beyond which the solid will be used.  Similarly, a solid with the shape of a good soft plains game bullet will be useless as a solid, which should drive deep and straight.  It is a high price to pay to have similar trajectory and wind drift.

The best scenario is to assume a very short trajectory for both and to zero the rifle at 100.  Both bullets will rise in the path from the muzzle, go over the line of sight at the close zero point, reach a high point in the trajectory and then drop to the line of sight at the far zero point.  In the bullet rise phase, the paths will be close together.  Divergance of the bullet paths get greater as the distance increases. There is nothing to be done about wind drift as this is a function of the difference in BC.

The total solution is to optimise bullet shapes with a good solid, a good plains game bullet and have two sighting systems for the rifle.  Either two scopes with quick detach mounts or a scope with quick detach mounts and iron sights on the rifle.  It is then a simple matter of sighting in a scope at 200 with the plains game load and then the second scope, or the iron sights, at 100 with the solid load.  There are no compromises and each type of bullet will give optimal performance to do best what it is supposed to do.

As usual, cheap solutions do not exist - one rifle with two sighting systems is cheaper and easier to transport than two rifles. 

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