Symbolism, Imagery, Allegory
At Tara, Scarlett shoots a lone Yankee raider, using the pistol her first husband Charles "had worn, but never fired" (26.18). Charles died of disease almost immediately upon getting into the army; he never fought in battle.
Charles is presented in the novel as a mooncalf-like, feminized drip. The pistol is a symbol of his masculinity, or lack thereof—it's a phallic symbol, which he never used, because he's not much of a man.
Scarlett, on the other hand, is quite man enough to use the pistol. She does so decisively, and though she's a little shocked at first, her most powerful reaction after killing the guy is to feel "vitally glad with a cool tigerish joy" (26.25). She's a warrior who has picked up her useless husband's gun, and taken his place better than he ever managed himself.
The Yankee has money, which allows Scarlett to keep Tara going; Rhett even tells her later that "murdering that Yankee […] really gave you your start" (57.127). Picking up the pistol allows Scarlett to become a success in a world where being a success is seen as, by nature, manly. She becomes her first husband… and she's better at the gig than he ever was.