The hit man is in his hotel room; he threads a silencer onto his revolver. A silencer will not work with that type of handgun.
McGarrett finds a used shotgun shell casing at the scene of a murder. The hit man who committed the crime used a double barrel shotgun which does not eject the shell(s) until the breech is opened. Professional hit men would never leave a shell casing behind and there was no reason for him to open the breech.