Self chambering gun

Question
Say a gun were created that could chamber a round with just a pull of the trigger instead of manually pulling the slide, what would the advantages of this be, say in law enforcement or military uses if there even are any?

Answer
What you are describing is called an open bolt gun, and they have existed for about a century. Their advantage is that they are cheap to produce. The disadvantages are myriad, which is why you don't see them in serious military service except in third world countries and in some special applications in first world countries.