The Dying Will Gun were first used by Vongola VII, who had the weakest Flames 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 used them to store his 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 super-powerful blast. It had the highest destructive and penetrative power of the Bosses' Weapons. Xanxus is also capable of using these to fly by blasting himself away from the ground.