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.