We always crab about the relative impotence of Martian heavy weapons. Yes, they have some random artillery that can be effective it actually hits, but the chances to hit are lower than Earth weapons.
One gun that everyone fears is the Martian lob cannon. These were massive mortars that fired a huge stone that was so large that if it struck an air ship it would cause an immediate trim critical. I managed to pick up a couple of the steampunk cannon by JR Miniatures. These are ground mount lob guns that could be used as anti-air batteries, or for bombardment. Pretty cool. They're cast in light resin, and put them on stone platforms. They look like ACW mortars on a massive platform, but aren't ornate like some similar Games Workshop guns, and their relatively cheap prices made them bargains.
Of course, every big gun needs some smaller guns to protect them. Of course there are no Nordenfeldts or Gatlings in the Martian armory, so I decided it would be quite possible for them to have a multi-barrel, slow loading weapon like an organ gun or ribaulequin. So I picked up a couple of the RAFM organ guns and they're ready to go. I haven't quite invented a rating yet, I'll have to come up with something. I'm thinking two dice per barrel, but they only fire every other (every third?) turn.