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.
A direct hit from the ball of energy will also deal a high amount of damage, and hitting a target's middle to upper body will usually result in the ball dropping straight down before exploding, making it a good way to hit specific targets with the explosion.
When fired into water, the projectile will continue normally, bouncing off any obstructions under the water. It will explode normally like other explosives.
pcps3xbox360 When taking the cannon from a drone, it has no icon (PipOS Error - 26 [File Not Found]). This is not necessarily a bug and could in fact have been included deliberately, due to the weapon's alien origins and the 'PipOS' not recognizing it. [verified]
pcps3xbox360 When examining a cannon dropped by a drone in the normal worldview (not by searching the drone's inventory), it will always show a value of 999 caps (app. half CND) regardless of the weapon's actual quality. This could also not be a bug and be deliberate, since the 'PipOS' not recognizing it and giving it a default value in caps. [verified]
xbox360ps3 When examining the drone cannon while in a guardian drone's inventory, the condition, damage and value are at its highest (Full CND,152 DMG, Value:1999 caps) but when after examining that drone cannon in your inventory, it will show it's true statistics e.g. 3/4 CND,129 DMG, Value:1357 caps. [verified]
xbox360 If you equip a drone cannon while wearing enclave tesla armor, the coils on the back of the chest piece will disappear. [verified]