The drone cannon is a weapon of alien origin which is added to Fallout 3 in the Mothership Zeta add-on.
It is the removed arm cannon of a guardian drone. It fires a ball of energy that acts as a grenade, bouncing and then exploding after a period of about 3 seconds. The projectile has the ability to move buses and cars before exploding. This weapon is hard to use at first, but once you understand its operation, it becomes an extremely devastating weapon.