@Durin, we were talking about megaprojects a little while back. I'm not sure if this counts or not, but it'll be useful. Here is the picture I drew of my idea:
The thing in the middle is an example city. You can tell because of how it's labelled 'City'. Around it is a railroad track that encircles the city. On those railway tracks are giant flamers. The idea is that these flamers will spew forth a very large amount of fire on ground forces attempting to attack the city. They are on tracks so that they can be moved and attack other people. The final design will have more than four flamers, obviously, it's just that I didn't want or have to draw them all. The entire system will be underground when not in use, preventing it from getting damaged by the wildlife or the (presumably) psychic weather of Avernus. Its prime use is in taking out large numbers of ground enemies, such as the orks which will attack and the daemons of the First Daemonic Incursion. Since the flamers are on turrets, they can only attack low-flying aircraft. Since they're underground, the enemy won't know about them until it's too late.
Let's look at other potential weapons and why many of them are unsuitable.
Melee weapons: Too short-ranged
Melta: Ditto, also incapable of continuous fire
Single solid shot (EM): We don't have the technology
Single solid shot (propellant): Perhaps may be used to kill all enemies in a straight line that are too far away for the flamers, so possible alternative to flamers
Single explosive shot: The turrets mean they can arc and hit enemies from far away, so possible alternative to flamers
Rapid fire propellant solid/explosive shot: Heat build up is a big issue considering the Assault Cannon
Rapid fire EM solid/explosive shot: We don't have the technology
Grapeshot: Inferior to flamer in all regards
Plasma/las: Heat build up is a big issue. Flamers operate at a much lower temperature than them, so continuous fire is possible. (Note: We should look into phased plasma fusils to get plasma weapons as they never overheat)
Haywire: We don't have the technology and not all of our enemies are mechanical
Missiles: Our cities already have as many as they can hold, which is a lot
Grav: We don't have the technology
Psychic: No
The ammunition for the giant flamers is a non-issue. Flamers use Promethium, and we have endless amounts of Promethium that we never use.