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Deflection is a technique used for effectively propelling a projectile at a moving target, also known as leading the target, i.e. shooting ahead of a moving target so that the target and projectile will collide. This technique is only necessary when the target will have moved a sufficient distance to displace its position during the time the projectile would take to reach the target's range. This can become the case over long distances (e.g. a distant target for a skilled sniper), due to fast moving targets (e.g. an opposing aircraft in an aerial dogfight), or while using relatively slow projectiles (e.g. a crossbow bolt or a basketball thrown to a running teammate).
Modern day fighter aircraft have automated deflection sights, where a computer calculates lead and projects the solution onto a head up display (HUD). The visual assistance with targeting the gun is offset by the enormous speed and agility of modern aircraft, compared to the days when targeting was less advanced.
In artillery, deflection is also used against fixed targets to compensate for windage and range. Due to the Earth's rotation, surface points have different velocities and curved motion, leading to apparent Coriolis drift of a long-range target.
Leading targets is the practice of aiming one's weapon ahead of the target so that the projectile will hit its mark. Over reasonably short ranges, leading is typically unnecessary when using firearms, but it is still relevant for sniping where the bullet may take a second or more to reach its target, as well as for weapons such as bows that use lower-velocity projectiles. It is generally unnecessary for guided projectiles, although the autonomous guiding mechanism may be designed to calculate a flight path to lead its targets on its own to ensure an interception.
Modern computer games of the first-person shooter genre typically feature a number of relatively low-velocity projectile weapons such as unguided shoulder-launched missile weapons, such that leading targets is necessary. Additionally, in some multiplayer games that calculate behavior on a remote server, even for high-velocity or hitscan weapons, there are issues of latency that may require leading. Essentially, even if the shooter has the target exactly in their sights, by the time the information relating weapon fire from the shooter's computer has reached the server, the target may have moved enough to avoid the shot.
Modern game-engines use a lag compensation system which moves all players back to a point in time based on the shooter's client interpolation time and ping (or more commonly by trusting the result detected on the client side) to evaluate if there was a hit or not. Such systems eliminate the need to lead hitscan weapons, but introduce the risk of players being shot after visibly taking cover.