Gyroscopic (spin) Drift
First, a quick fact about spinning objects…
Picture a spinning object like a bullet or a top. The spinning thing has a ‘spin axis’, about which it’s spinning. If you try to disturb the spin axis by applying a force, or a torque to that axis, the spinning object reacts in a strange way. Rather than simply moving in the direction that you pushed it, the spin axis reacts by moving 90 degrees from the applied force, in the direction of rotation. In other words, if you have a top spinning clockwise on the table in front of you, and you push the top of it’s stem straight away from you, the stems first reaction is to jump to the right. After the initial reaction, it will precess into its new equilibrium. Now, on to bullets.

Consider a bullet fired at some angle on a long range trajectory. The bullet starts out with its spin axis aligned with its velocity vector. As the trajectory progresses, gravity accelerates the bullet down, introducing a component of velocity toward the ground. The bullet reacts like a spiraling football on a long pass, by 'weather-vaning' it's nose to follow the velocity vector, which is a nose-down torque. The price you pay for torqueing the axis of rotation is that the nose points slightly to the right as it 'traces' to follow the velocity vector. This slight nose right flight results in a lateral drift known as ‘gyroscopic drift’.

Having a left or right twist will change the direction of gyroscopic drift. Bullets fired from right twist barrels drift to the right, and vise versa by the same amount, typically 8-9 inches at1000 yards for small arms trajectories. Gyroscopic drift is an interaction of the bullets mass and aerodynamics with the atmosphere that it’s flying in. Gyroscopic drift depends on the properties (density) of the atmosphere, but has nothing to do with the earth’s rotation.