Keeper's book has "Machine Gun" specialization for automatic fire from a bipod or tripod-mounted weapon. An automatic rifle that wasn't deployed and used like that would actually use the "Submachine Gun" specialization when firing auto and not on single shot. Given this, "Machine Gun" seems perfectly reasonable for something like a Lewis gun.
For an actual cannon, the weapons tables include a 120mm tank gun for the modern era (ala the 120mm L/44 main gun on an M1A2 Abrams) and associate it with the Artillery skill. So, it would be reasonable to include large-bore vehicle cannon, not just field artillery or mortar tubes etc.