Eh. In my book, artillery points up. It doesn't, it doesn't counteract gravity effectively, its range is insufficient. I mean, yeah, you could make an artillery piece that's horizontal and fires a long distance, but considering the amount of propellant (and thus force) you'd need to get it to go a decent artillery range... really, why would you? Anything you hit with an artillery shell is gonna explode the same no matter what angle you get it from...
Humvees aren't armored either. Doesn't mean they aren't mounted with those hugeass antipersonnel guns and used in combat.