Judge Dredd: In case you have forgotten, this block operates under the same rules as the rest of the city. Ma-Ma is not the law. I am the law.
When Judge Dredd says: "Thanks for the heads up", you can see that the gun he is holding is a rubber stunt prop: you can tell because the front end is sealed shut when it should be open. Also, the barrel is thicker, certain details are missing, etc.
Dredd's count outside the medical center is unusually perfect for time in a movie: there are exactly twenty seconds between his warning and the first shot.