Loaded up several cannons with Uranium HE (what I currently have a stockpile of) shells behind my defenses and it obliterated anything in its line of fire, including the walls. Is this the intended behavior?

Yes, this gun uses the original tank shell, and the effect is the same as the original tank gun. It will destroy any entity in the path, including walls and stones. I recommend you to use my new rocket turret https://mods.factorio.com/mod/hl-Rocket-turret

Ah, I am fairly new to the game, just started second playthrough.

Have a good time.

You can use https://mods.factorio.com/mod/SmartCannonShells with this to have cannon shells that do go over walls