player to player interaction

[skillmaster19]

How would I make it so the actions of one player trigger someone elses? For example if a player fires a weapon all players within a 8x4 area of the players direction are effected.(like being hit by a sword).

Another example would be a slap item that causes the player next to the player that uses the weapon to say "owww, that hurt!" I understand there are projectile functions, but I want to know the best way to immediatly effect the players within a certain area.