This is not for codex marines, don't worry. However some variant list might want to use it which I must consider.

Note that cannon might be AP4+/AT6+ shot instead of BP. Since it does resemble Thudd Gun a lot.

Thudd Guns are used by the Imperial Guard, not Space Marines.

...don't try to emulate the 40k game in it's awful current form.

40k has been falling exponentially from a wargame to a collectible miniatures game- with the intrinsic Codex Creep, new models having uber rules, etc in the noble quest of holding up a flailing business in a collapsing economy. Each new release is more and more shameless in it's lack of balance, and attempt to get players to shelve their existing models and buy the new novelty toys.

Next you'll be telling me Epic Formations must have Special Characters that for less pts signifincantly boost everyone in the army....shudder. "All my Tactical Formations have Marneus Calgar..."

I'd much rather Epic went it's own way in terms of balance and cohesive armies- rather than pandering to the whim of 40k and the youngsters with parent's wallets it's aimed at.

Marines used to have Thudd Guns, Molemortars, Rapiers and Tarantulas. That was back in RT times. Nowdays they don't except that there was somewhere a stat for Tarantula for Marines (don't remember where though it was in army builder).

Marines do have Tarantulas in the current 40k; Forgeworld sell those, in Multimelta, Lascannon or Heavy Bolter configurations.

Those would probably show up in a Marine "defence" list though, are they even mobile on their own?