After the last round, the bolt carrier is held back and you cannot pull the trigger. After you press the bolt catch and release the bolt carrier, you can pull the trigger.
If you fire the last round with the magazine inserted the gun will lock in the open bolt position via the bolt catch. Any subsequent trigger pull(while the bolt is retracted and held open) will not release the hammer(whitch is being held under the the bolt carrier) but will allow the trigger to go through it's forward backward travel. After releasing the bolt carrier(bolt release or charging handle) the hammer will be caught by the trigger sear and you will be able to fire the gun with the trigger again(if it's not on safety ) I second the google comment!!!!! ps to answer your original question ....... they both do absolutely nothing as you pull the trigger with the bolt catch engaged !!!!!!!!!