So the Light Artillery... What does it actually shoot? I have it loaded with grenade launcher ammo because that's what it would take but it doesn't shoot. I also don't have the artillery aiming tool to try manually.. Am I missing something?

yes. You need to craft "Light Artillery Targeting Remote" for manual targeting.
Also, the auto-fire range is only 90 tiles. That's three chunks. You might be trying to destroy something at too great a distance.

I'm pretty sure the range isn't a problem. There was lots to shoot at at different distances.

Then I can't tell you. In my test save, it shoots both automatically and manually.