"Bump firing is the act of using the recoil of a firearm to fire multiple shots in rapid succession. This process involves holding the foregrip with the non-trigger hand, releasing the grip on the firing hand (leaving the trigger finger in its normal position in front of the trigger), pushing the rifle forward in order to apply pressure on the trigger finger from the trigger, and keeping the trigger finger stationary. The firearm will recoil and then return to its previous position after the round has been fired first resetting and then pressing forward against the trigger thereby firing successive shots."
"All these techniques horrendously degrade the accuracy of the firearm, due to the necessary jerking of the weapon, which makes viable aiming impossible. The techniques trade accurate, aimed fire for an increase in the firearm's rate of fire. Bump-firing requires practice and concentration. It is uncommon to "empty the mag" without at least one stoppage. The inaccuracy, difficulty, and ammunition costs render the practice uncommon."
Someone who actually is aiming with a semi-automatic is almost always going to be more effective than someone firing in nearly uncontrolled bursts. It is very difficult to aim while bump firing, this may be true with automatic fire, but bump firing makes it even worse AND even most automatic weapons are used in short controlled bursts.