Audio problem: While at the range, Jamie takes Frank's revolver to the line and fires it. The last 2 shots you can hear the casings hitting the ground, but not on the first 3 shots. The last 2 shots are from another camera angle. Revolvers don't eject the casings, so we shouldn't hear them landing.
Factual error: Danny shoots Frank's revolver, and as he continues to shoot, you hear the sound of a casing hitting the floor, which shouldn't happen since it's a revolver being fired. (00:11:46)
Answer: It's a common cop show plot device. If the police on the show always acted in the most logical way (i.e. not alerting a suspect to their presence) the show wouldn't have the opportunity for an exciting foot chase.