Weird one, but here we go.
-A modern world, with the same laws of physics/same materials, weapons, etc.
-The player is a normal human. Think baseline average American citizen that has a normal physique.
-The player is able to use RPG-Leveling elements to level up.
-The player can increase his Endurance. Each point in Endurance extra is another 1% efficiency of the human body at preventing injury. (So, 1% better at stopping cuts, enduring heat, stopping blunt trauma, etc).
Question: How many points in Endurance would the player need to have before 9mm pistol rounds stop being lethal and turn into mere annoyances? (Where the skin/muscle can negate/stop the force of the bullet)