The Fission Cannon appears as normal capital-class railgun, either mounted internally - usually forward-facing, or in a large turret. Due to the relative fragility of the railgun, the internal mounting, with it’s greater potential for armoring is more common.
Full Item Description
The Fission Cannon is a generally a standard capital-ship railgun modified to fire a special projectile. This projectile is a large slug of transuranic matter as well as softer dampening materials (the exact formulation is classified) that is sufficiently unstable to initiate nuclear ignition upon impact with an enemy ship. Yields vary, but being a pure fission device, is generally limited to sub-megaton.
The ammunition is highly unstable and has a very short half-life, so ships using this weapon system also need to be installed with a purpose-built reactor that produces the necessary material. The exact composition of the slugs can be varied to take into account various conditions, targets, etc.
The weapon system, supply infrastructure and breeder reactor all need to be heavily rad shielded and armored against battle damage. Very large ships, such as the Ballista class battleship, may deploy multiple breeder reactors.
These systems are best used against armored warships, as thin-skinned ships might not provide enough resistance to cause the slugs to properly compress. In those cases, the rounds do little more damage then standard depleted uranium or osmium slugs - punching completely through the vessel.
They are most lethal against moderately armored ships, as the projectiles may punch fully through one layer of armor and then detonate when they strike the other layer on the way out, causing an internal nuclear detonation.
Against heavily armored ships, generally those with more then 1 meter of hard armor, the rounds surface detonate, destroying any external structures within hundreds of meters and biting deep into the armor.
Other options are to fire rounds with varying velocities to allow for collisions or time on target strikes. It is possible with a sufficiently accurate system to have projectiles strike each-other, causing a detonation without striking an enemy ship. This could be used to attack groups of smaller vessels such as fighters, which are too small and unpredictable to directly target.
Due to the sub-light velocity of the projectiles, they are considered a short range weapon and it is highly unsuitable for planetary bombardments against worlds with signficant atmospheres. Detonation is affected adversely both by loss of mass due to atmospheric friction, and loss of velocity.
In its effective performance environment, short-range ship to ship combat, the rapid cycle time of modern capital ship railgun allied to the nuclear payload allows an unprecedented amount of energy being delivered to enemy targets.
One advantage of this system is it allows for reactor energy to be stored (short term!) in the unstable nuclear slugs. As a result, a smaller ship can match the firepower of larger ships whose weapons feed directly from current reactor output. Batteries and capacitor systems can produce similar capabilities, but these generally cannot store energy with the same density as nuclear materials.
And yes, if antimatter is commonly available, this system would be superseded.
This idea was inspired by descriptions of ships in the TTA series of books. They mentioned the weapon Nuclear Pellet Launcher without any details on one actually was. So this is my take. http://www.bisbos.com/rocketscience/tta/toad.html
Ballista class warship
The Ballista was the first warship class built after the development of the Fission Cannon that was specifically designed around this weapon system.
The Ballista has a long, slender profile, with a simple configuration. The front two-thirds of the vessle comprise six fission cannons arranged radially around the ship’s primary axis with thick armored shrouding. The primary sensors are also present in raised blisters on port and starboard sides of the bow. The aft third of the ship is devoted to the engines, main powerplant, and the three breeder reactors used to supply the main armament with ammuniton. In addition, three heavily armored pods attached to the main hull by thick nacelles contain the command and control systems, sensors, secondary armament and crew quarters. All three of these pods are detachable and can navigate independantly. The command and control unit is able to operate the ship even when detached.