Specifically, Smith & Wesson stopped producing the Model 29 several years before the film's production. Clint Eastwood himself contacted S&W representative Bob Sauer, who had several assembled from parts at the factory. Smith & Wesson reintroduced the Model 29 shortly after the film's release.
Throw It In: Andy Robinson improvised several of Scorpio's lines, including the "hubba hubba" bit over the telephone and "my, that's a big one" when Harry pulls his gun in the park. The flip that Scorpio does when he is shot by Harry in the stadium was also Robinson's idea.
Frank Sinatra was intended to play Harry, but he had a broken wrist at the time and felt he could not possibly hold Harry's gun.
The screenplay for the first film went through numerous iterations and had several potential endings, including one in which Scorpio gets dispatched by a Marine sniper rather than Harry himself. Eastwood felt that the later drafts were overcomplicating the core premise and opted for one of the earliest screenplays written instead.
Audie Murphy was initially offered the role of the Scorpio Killer, but declined due to fears of scarring young children.
Working Title: The title of the first film's script was Dead Right.