It seems some types of shell-firing turrets can't hit enemies beyond a given range when using standard (non-instant-hit) shells, because they detonate/impact the ground at a set maximum distance; said distance can be shorter than the minimum range in the Heavy varieties. I tested this with all six shell-firing turrets, all four kinds of vanilla cannon shells, with and without other mods installed.
Example of the issue: https://i.imgur.com/0iihtZ9.png
It would seem the cause of this is that shells as ammo, unlike firearm magazines and rockets, have a set range of 30 tiles. This means the Light and Dual frontline cannons can use them without issues; the backline ones start having problems; and both Heavy varieties can't use them effectively at all. This mod's "light shells" don't have this issue due to their particular mechanics, and they can be used in all the shell-using turrets without any problems.
If there was such a thing as a range modifier for guns in the api this could probably be easily solved, but I can't find anything like that. The only other thing I can think of to prevent newcomers to the mod being confused with the ineffectiveness of cannon shells is to create a new ammo category for your light shells and have the turrets use that category instead, but it doesn't seem like a perfect solution.
