Wikipedia:
In the scene in which Lee was accidentally shot, Lee's character walks into his apartment and discovers his fiancée being beaten and raped by thugs. Actor Michael Massee's character fires a .44 Magnum revolver at Lee as he walks into the room. A previous scene using the same gun had called for inert dummy cartridges fitted with bullets (but no powder or primer) to be loaded in the revolver for a close-up scene; for film scenes which utilize a revolver (where the bullets are visible from the front) and do not require the gun to actually be fired, dummy cartridges provide the realistic appearance of actual rounds. Instead of purchasing commercial dummy cartridges, the film's prop crew created their own by pulling the bullets from live rounds, dumping the powder charge then reinserting the bullets. However, they unknowingly or unintentionally left the live primer in place at the rear of the cartridge. At some point during filming, the revolver was apparently discharged with one of these improperly-deactivated cartridges in the chamber, setting off the primer with enough force to drive the bullet partway into the barrel, where it became stuck (a condition known as a squib load). The prop crew either failed to notice this or failed to recognize the significance of this issue.
In the fatal scene, which called for the revolver to be actually fired at Lee from a distance of 3.6–4.5 meters (12–15 feet), the dummy cartridges were exchanged with blank rounds, which feature a live powder charge and primer, but no bullet, thus allowing the gun to be fired without the risk of an actual projectile. But since the bullet from the dummy round was already trapped in the barrel, this caused the .44 Magnum bullet to be fired out of the barrel with virtually the same force as if the gun had been loaded with a live round, and it struck Lee in the abdomen, mortally wounding him.
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