Newtons three laws of physics are fundemental to understanding what is heppening when a hand held weapon is fired.                            

                                    F=ma
  
     When a gun is fired, the explosion puts a force on both the bullet, and the gun/person system.
    Becuase of the small mass of the bullet, its acceleration is very large, while the large mass of the gun/person system means it experiences a very small acceleration.
    This puts a limit on both the size of a projectile that can be fired out of a gun, and the power of the explosion used to accelerate the projectile, because a person can only deal with a certain amount of force.