Originally posted by: JohnCU
how is that possible?
Well, there are certain models of Glocks that are manufactured for Law Enforcement use and are capable of fully-automatic (selective) fire as they come from the factory. The extended magazine used in the video clip was designed for such a weapon.
With respect to "civilian" Glocks, it's a fairly simple matter for a skilled machinist to
become a felon by modifying the pistol to fire "full-auto". Usually what is required is a special modification to the upper receiver, so that the hammer (or, in the case of the Glock, the striker) is released just as the slide returns to battery and locks the breech. The timing is
very critical -- if the round is fired too soon, the breech will not be locked and the gun will probably explode. Kids, don't try this at home...

More details can be found in
The Glock Exotic Weapons System.