Obscure ballistics force of the day...

In external ballistics a number of forces act on bullet flight, making for a royally nasty set of differential equations to solve. The basics, such as gravity and aerodynamic drag (including crosswinds), are conceptually pretty straightforward. One that I find interesting for some strange reason is the Magnus effect. The magnus effect is what makes curve balls curve. Topspin on a ball makes it fall faster than it otherwise would (provides "lift" downward), backspin makes it fall more slowly than it otherwise would (provides "lift" upward).

Things get interesting when dealing with bullets. Since they spin around the axis of travel, the aerodynamic forces caused by the spin cancel each other out. However, add in a crosswind, and things get interesting.

Take your standard bullet fired from a right-hand twist barrel. (Imagine a barrel, take your right thumb, point it along the direction of your imaginary barrel, and curl your fingers toward your palm. The bullet, which is traveling in the direction of your thumb, spins in the direction your fingers curl.) Looking from behind the bullet, the spin appears clockwise. (The right side is moving down, the left side is moving up.)

Now, add in a cross wind coming from the right.

The wind hitting the bullet from the right sees topspin. The right side of the bullet is spinning down, creating "lift" to push the bullet downwards. (like a curveball) This causes the bullet to fall faster than it otherwise would, causing the bullet to hit the target lower than it otherwise would.

A wind from the left sees the bullet as having backspin, which has the opposite effect. The spin creates lift, and the bullet falls more slowly than it otherwise would, causing the bullet to hit the target higher.

The magnus effect on a normal bullet is small, small enough that it is almost always disregarded in small arms ballistic calculations, (Field artillery is another mater.) but, for some horribly geeky reason, I find it rather interesting.