Magnum

 is a gun. It is a 6-round revolver that deals decent damage with a high fire rate.

Trivia

 * "5 shots or 6?" references the film Dirty Harry.
 * The gun's description also is a reference to Harry calling his magnum a filthy gun.
 * The second part of the description might be a refence to the Gungeon itself, since the gun that Bullet Kin hold actually looks like a Magnum. A Bullet Kin might have abandoned or forgotten it where the player finds it.
 * The synergy may be a reference to McCree from Overwatch, who has the ability to reload his revolver while performing a roll.
 * The synergy may be a reference to the Gentlemen Bastard series by author Scott Lynch in which the Wicked Sisters are the twin hatchets serving as the primary weapons of one of the series’ two protagonists - Jean Tannen.
 * The Magnum's altered form with the synergy was originally an unused standalone gun, also named Wicked Sister.