From the preview:
The Surtr Heavy Heat Cannon is a new resin (as in traditional resin resin) upgrade kit to the Jotunn Heavy Hailstorm that not only adds a new Barrel to this excellent artillery piece, but also a support platform and crew for use with the spare gun. Either gun will fit, so you can keep the Jotunn Heavy Hailstorm on the main carriage and have the Surtr Heavy Heat Cannon on the platform if you so which.