The Core Mechanics (from rules of play)

   Every single game has a core mechanic. The core mechanic is a very important play activity that players experience and perform again and again throughout a game.
   So from what I have read a core mechanic of a first person shooter game would be a set of related iterative actions the player conceives while playing the game. For example such as moving, aiming, shooting and also rationing health, stored ammo and armor. If these actions are not repeated or carried out in play the player won't promotionally move on to a higher rank or level. In a abbreviated matter of speaking the player won't progress.