This is what happens when you are so hasty and excited to get something in order and haven't taken the time to read and study the issue properly beforehand.
The artillerist as an NPC already exists in the DMG with abilities and costs provided for. This entire page was unnecessary but I am going to keep it here as a reminder to myself to ensure due diligence in the future.
An Artillerist is a sub-class of Fighter. I recommend this class only for Henchmen or NPC's as I have not yet play-tested it as a PC class.
The Artillerist is a specialist in heavy artillery and munitions/explosives. As a melee fighter, they are rather limited. The Artillerist uses the Fighter table in terms of advancement, hit dice, etc...
Arterillerists need high scores in strength, dexterity and intelligence, requiring a minimum of a 16 in all of them.
The Arterillerist is proficient in using missile weapons in general, such as the large wagon bound weapons, but are also highly proficient in all hand held missile weapons as well. So much so that they gain a +1 to hit/to damage at every 3 levels of experience.