The so-called energy cannon, of course, does not use gunpowder as its propellant.
In Tom's weapon system, the energy cannon corresponds to the energy gun.
The energy gun fires small-mass projectiles that have no propulsion; once fired, their trajectory is fixed.
The energy cannon, however, fires shells that possess self-locking capability and propulsion, allowing them to track targets in space. Because of its similarity to the artillery of Earth's past, Tom adopted the name 'cannon'.
Its initial launch propulsion is still electromagnetic force, relying on the same mechanism as energy gun projectiles to gain initial velocity under electromagnetic force and exit the barrel rapidly.
