The shotgun that Napoleon Wilson gets can only hold five or six shots. Now watch the big gun fight scene where the street gang are breaking in. Wilson clearly fires at least nine or ten shots with the shotgun before reloading it. Also, watch
the part with Wells. He has a revolver, in which he fires eight or ten bullets out of the six shot gun.
In reference to the scene with Welles the reason we see him attempt to fire more than 6 six shots is the gun has a silencer on it. This is pointed out later in the story line by the female officer when she disarms him only to find his weapon empty. At that time he declares "damn i must have been shooting blanks all night". Still not a excuse for firing a gun more than able but crucial to the story.