I emailed FW for a correct base size for the Magma Cannon, as I am currently proxying the model until I purchase the proper model. From their picture on the site, I thought it was the same as the Iron Daemon (60x100) but their reply left me speechless.
For the Magma Cannon we would recommend a minimum base size of 200mm by 60mm (or two Screaming Bell base together)
I emailed them again just to be 100% sure that they were referring to the Magma Cannon & not the Dreadquake or something and they confirmed:
The model is longer and so would need the longer base.
Ironically, that wouldn’t be that bizarre - the most recently released Monstrous Beasts and Cavalry units from the newest books (Sepuchral Stalkers, Nercropolis Knights and Mournfang Cavalry) are all on chariot bases. So there’s precedent.
My guess is they confused it with the dreadquake mortar, which would do better on two screaming bells. I put it on two chariots, but it was a little too wide. The magma cannon and the Death Shrieker both fit on chariots just fine.