I don't know if anyone else has had this problem, but the shells don't fly over the walls I'm using. Kind of makes it useless for defense :(.

hmm... Do you used [cannon shell] or [explosive cannon shell] in vanilla??
or other mod's shells?

I don't change original and other mods entity.
Then I can only assure the function of my mod shells.

Please, I ask you to reaffirm which turret and shell you used.

Oh. Yeah, I was using the "cannon mk2", and I was using the "improved tanks shell" from the "Tanks! For Bob's mods! mod

I will look for ways to control the shells in other mods.
But I do not know if it is possible.

If you want a shell over the building, it will be the fastest and most sure to ask the maker of the mod.