This is generally more difficult than hitting a stationary target. You have to try to predict where the target will by the time the projectile (whether it be a bullet, a snowball, or your fist) gets there. This can be even more difficult when shooting a rifle, since it's very challenging to judge the speed of a bullet traveling at supersonic speeds.

You also have to judge the effects of wind and gravity, as with any other shot. This can prove difficult, especially if you are a sniper and only get one shot. Being a sniper is more challenging than one might originally think.