In the campaign, the fuel rod cannon first appears in Blue Team, found in the coolant pipes area and in the Prowler hangar. It later appears in all later levels where Covenant appear as enemies.
Warzone, Arena, and Campaign feature two additional variants:
Light of Urs: an improved Fuel Rod Cannon with faster moving projectiles that can be fired more quickly, have a larger blast radius, and can lock on to vehicles at the expense of a slight damage reduction. This weapon is more useful for anti-infantry engagements, but is still otherwise somewhat effective against large targets. It is also available in the mission Genesis, where it comes in handy against Promethean Knights, enemy Wraiths, and the Warden Eternals.
Pool of Radiance: an advanced Fuel Rod Cannon with a 40% lower rate of fire compared to the normal weapon but the fuel rods leave behind explosive submunitions, have a 42% larger blast radius, and feature a proximity airburst fuse. This weapon is very useful against bosses (e.g. Warden Eternal) or other slow-moving/large targets (e.g. Scorpions). It is also available in the mission Reunion, where it comes in handy against the Warden Eternal and his Prometheans, especially if paired with the Talon of the LostNeedler, even on Legendary difficulty.