It was a real gun, not a trick gun. The Joker used it to kill the guard when he walked into Harvey's hospital room.Originally posted by: bunnyfubbles
Joker wasn't exactly suicidal, he wanted Batman to break his rule and hit him (thus more than likely killing him). He didn't necessarily want to beat Batman, he just wanted to break him...he wanted to do the same with Harvey, although I get the feeling he could have easily used a a trick gun in case Harvey actually did pull the trigger...of course I don't think we'll ever really know for sure, but its probably best to take that scene at face value.
The Joker kept saying, "I'm a man of my word," and I don't think he ever did anything to prove otherwise.