As before, the modifications available are either a precision scope for long-range fire, or a micro-missile launcher which launches small missiles with a narrow seeking angle: these explode with a small amount of splash damage a short time after impact. The scope functionality has been changed somewhat since the previous game, however: it is a drastically slower single-shot mode, and uses six ammunition per shot when aimed.
The micro-missiles are essentially the same, save that there are now launchers on both sides of the barrel for a total of 12 shots between loading pauses. There is no longer an upgrade to reduce ammo consumption, as this has been replaced with a new "Primary Charger" upgrade. With this, a kill with standard gunshots from the Heavy Cannon (with the Micro Missile mod equipped) will supercharge the weapon, causing it to glow red for five seconds. In this state missile damage is increased by 30%: with the "Bottomless Missiles" Mastery, this results in the Heavy Cannon being able to deal more damage per second than the Chaingun in Mobile Turret form.
As with most other weapons in Doom Eternal, the Heavy Cannon has several weapon skins available.