This is from Wikadia (sp)
"The gun used by Clint Eastwood in the filming of the movie was reportedly not a Model 29. According to a story related by a member of the studio's prop department, Smith & Wesson did not have a Model 29 in stock at the time one was requested for filming. Instead, they used a Smith & Wesson Model 57 in .41 Remington Magnum. The Model 29 and Model 57 are identical except for minute differences in bore size, chamber dimensions, and exterior markings, none of which are visible in the film.
However, in the final scene between Callahan and Scorpio, there is a close-up of Callahan's revolver. A freeze-frame of this close-up clearly shows the markings "44 MAGNU" with the final "M" not readily distinguishable, due to pixel resolution limitations. Additionally, a careful comparison with a Smith and Wesson Model 629 (the stainless-steel version of Callahan's Model 29) shows the bore of the muzzle to be the same proportional size (a .41 caliber, while large, is visibly smaller to a trained eye). Thus, while it is certainly possible that other N-frame Smith and Wesson revolvers were used as props throughout the film, this scene clearly used a genuine Model 29.
Another version of the Dirty Harry gun story: In the scene where we see Inspector Callahan drawing his oversized revolver for the first time (bank robbery scene), the gun used was a Smith & Wesson Model 29 in .44 Magnum with a 6 1/2" barrel, but at the entrance to the alley when chasing the man with the tan suitcase, the model with the 8 3/8" barrel is clearly visible. Subsequent shooting (both with the cameras and the gun) was conducted using a Smith and Wesson Model 25 in .45 Long Colt with a 6" barrel. The choice of .45LC over .44 Magnum was to use the standard "Four-In-One" blank cartridges which were widely used in filming cowboy movies and thus, readily available (unlike blanks for a .44 Magnum, which the prop department would have had to fabricate from scratch). As in the story above, the Model 25 and Model 29 are so similar that it would be impossible to distinguish them at a glance. However, the film's co-writer, John Milius, who owns one of the original guns from "Dirty Harry" and its sequel, Magnum Force, has written in gun magazines that a .44 Magnum was indeed used for filming and publicity shots.
The 44 Magnum cartridge has since been eclipsed as "the most powerful in the world." Factory-produced examples include the .454 Casull, the .475 Wildey Magnum, the .50 Action Express, the .460 S&W Magnum and the .500 S&W Magnum. Some gunsmiths also offer custom-built or limited-production handguns chambered for proprietary calibers or high-velocity rifle cartridges. Perhaps the most powerful handguns ever made are single-shot pistols chambered for the 50 BMG (Browning Machine Gun) and .600 Nitro Express calibers.
The gun Scorpio steals from the liquor store owner and uses in the bus hijack is a 9mm Walther P38. Scorpio also uses a Sten submachine gun in his rooftop shoot-out with Harry.:
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Bob