He actually used a couple of guns....
In the first movie, they called it a ".44 magnum" but the gun they actually used in most scenes was a model 25 in .45 Colt. If you look at the barrel in the shootout scene at the end of the film, you notice ALL the markings were ground off. It also used the old five in one blanks which Hollywood had been using in Peacemakers on western flicks for years.
Why the .45 ? This was to make the bore look bigger in scenes where you saw the muzzle........
For publicity scenes they did indeed use an 8 and 3/8 inch Model 29.
In the script that was turned in by Millius, he had Harry carrying a four inch nickel plated model 29. Later, he saw a four screw six and a half inch blued 29 at a gunshop he wanted and that was the gun picked for the film.
In the second film, the gun was fairly well known and they just used a 6.5 inch model 29 and left the markings on the barrel intact. They are obvious in the credits of "Magnum Force."
In Sudden Impact, Harry used the Automag.
Couple of years ago, I was reading the old novelization of the first film by Harry and Julian Fink and it dawned on me the film could easily be remade today. I kinda figured Hollywood would do something like cast pro-wrestler The Undertaker as Dirty Harry and arm him with the .500 magnum....
Ill be damned, about six months later Wrestlemania came about and they did these small advertisements spoofing varous famous movies ....One had the famed shootout scene from Dirty Harry and there is Mark Callaway (the Undertaker) dressed like Eastwood and saying the lines. Still had the blue 29 though....