Todd Roybark
Hero
Shatter states this:Then you have shatter which you would assume would be a good pick vs the cannon but it has clear text stating that it only effects non magical objects
A nonmagical object that isn't being worn or carried also takes the damage if it's in the spell's area.
So as a magical object, a DM can either rule the Eldritch Cannon is immune to the damage, or gets a Saving Throw at +0.
I, frankly would apply this rider of the Shatter spell as well to the cannon:
A creature made of inorganic material such as stone, crystal, or metal has disadvantage on this saving throw.
To me, it seems nonsensical to have a Nimblewright Construct be seriously threatened by a Shatter spell, but a steel wingless dragon shaped Eldritch Cannon, not even be affected.
The Eldritch Cannon has AC, HP, and a Saving Throw entry...which leads me to believe that the design intent was not that an Eldritch Cannon be a neutrino in the D&D material world.
XGE makes clear that a Shepherd Druid's Spirit Totem is indeed a neutrino in the D&D material world...via this line:
It counts as neither a creature nor an object, though it has the spectral appearance of the creature it represent
As an aside, nice to see you Stoutstien, haven't seen you post at ENWorld in awhile.