I also had heard that about the 41 Mag, but here is the straight poop-
Dirty Harry helped popularize the Smith & Wesson Model 29 revolver, chambered for the powerful .44 Magnum cartridge. The film initiated an increase in sales of the powerful handgun, which continues to be popular forty years after the film's release. The .44 Magnum ranked second in a 2008 20th Century Fox poll of the most popular film weapons, after only the lightsaber of Star Wars fame. The poll surveyed approximately two thousand film fans. However, the only appearances of the Model 29 in the movie are in the close-ups: Any time Eastwood actually fired the revolver, he was shooting a Smith & Wesson Model 25 in .45 Long Colt. In 1971, .44 Magnum blanks were not available. However, as a result of decades of Hollywood Western movies there was an ample supply of 5-in-1 blank cartridges. As the Model 25 is built on the same Smith & Wesson N frame as the Model 29, it was simple to substitute it for the Model 29 in scenes where Eastwood had to shoot the revolver.
The .44 Magnum used in the film is now owned by Prop Master and weapons specialist Bill Davis, who bought it from the production company before the film became popular. The revolver is still in use as part of his catalog. Director John Milius owns one of the actual Model 29s used in principal photography in Dirty Harry and Magnum Force. It is (as of March 2012) on loan to the National Firearms Museum in Fairfax, Virginia and is on display in the Hollywood Guns gallery.
What we have here is... failure- to communicate.