When this thread started, and I saw it, that seems to have been a myth.
"Dirty Harry's gun is supposedly a Smith & Wesson Model 29 revolver, chambered for a .44 Magnum cartridge. In the film, the gun is shown as being capable of sending assailants flying through the air, even when shot from a distance, however, in reality the gun does not produce such dramatic results. Additionally, the .44 Magnum round is not considered to be a practical caliber for urban police force use due to recoil (which makes target re-acquisition difficult) and over-penetration issues, which greatly increases the likelihood of the bullet going through its target and injuring bystanders. The actual gun used on set by Clint Eastwood was in fact a Smith & Wesson Model 29.
It is a common misconception that a Model 29 could not be located and a Model 57, chambered in .41 Magnum, was used instead. Clint Eastwood contacted Bob Sauer, representative for Smith and Wesson, to acquire the pistol. The Model 29 had been in production up until the late nineties, but a number of pistols were assembled from parts at the factory and provided to the crew. Eastwood took one to a firing range to familiarize himself with the Model 29."
Dirty Harry IMDB Trivia