Hitscan
With a game, hitscan describes a projectile with no travel time and is instantly registered as a hit on the cross-hair. For example, the bullets shot from most guns in a game are hitscan. However, non-hitscan weapons like an arrow, missile, or an RPG (Rocket Propelled Grenade) are slower and affected by gravity and other game physics. Weapons that are not hitscan require the player to predict where they believe the target will be when the projectile reaches its destination.
In some games, the distance a target is shot at can affect hitscan. For example, a sniper rifle at a close distance is a hitscan. However, you may have to account for travel time and other physics when targeting long-range targets. If a long-range target is moving left-to-right, leading the target by moving the cross-hair slightly to the right may be required.
What is bullet drop?
Bullet drop describes when a bullet or other projectile is affected by speed, distance, gravity, and other game physics. Games with a bullet drop effect require the player to aim slightly higher at long-range targets. When shot, the bullet's line of departure travels upwards, and while traveling, gravity affects the bullet's trajectory, causing it to drop downwards. If the bullet trajectory could be seen, it would appear that the bullet traveled with a slight arc instead of a straight line.
ADS, Bullet, FPS, Game terms, Gun, Ray casting