The Mustang .357 is a revolver manufactured in-fiction by the Duke Arms Company and is due to appear in Grand Theft Auto VI. It was spotted in an official screenshot, held across her partner's chest by a biker woman, with the Duke Arms Company branding visible behind the cylinder and the weapon's name engraved along the barrel. Its design and name both draw on real Colt firearms: it appears to be based on the Colt Python, its ".357" name nods to the Python's .357 Magnum chambering, and "Mustang" is borrowed from the separate Colt Mustang pistol.
Weapon data via GTA Wiki (Fandom), CC BY-SA 3.0.