The Dying Will Gun was first used by Vongola Settimo, who had the weakest Flame among all the Vongola Bosses. With this gun, he could store his Flame in it and release it as a more powerful and condensed energy. He used Dying Will Bullets as ammo and stored his Sky Flames within them.
Xanxus takes the idea of these Guns and creates his own version. He uses two guns instead of one, and the difference between Vongola VII and Xanxus is that Xanxus possesses and stores the Flame of Wrath within his X-Guns, creating a more powerful blast and hit. Xanxus is also capable of using these to fly by blasting himself away from the ground.