Little kids do it when they play cops and robber or cowboys and indians. Actors do it when they are acting.
Psychologists even use roleplaying as an aid in their profession.
Some may have more rules than others. Because pretend is freeform, there are no rules to determine whether or not the imaginary bullet struck its target. In most cases, there is a number that represents a chance for the imaginary bullet to hit. Of course, no one is really shooting or getting shot at. Roleplaying games take part entirely in the imagination of the people playing the game.