Before the Warforged were created and fully automatic, a group of gnomes worked on mobile armored platforms that they could ride inside and pilot. This project was later folded into the warforged project after a completely different experiment accidentally lead to the creation of an Artificial Mind that they could use inside the mobile platforms. Armorists were no longer necessary, but that didn't stop some from continuing the project on their own.

 

Restriction: Gnomes Only

Only gnomes can become Armorists. Your DM can lift this restriction to better suit the campaign, of course.

The reason for the restriction is because Warforged are medium creatures and I didn't want to answer questions of other races becoming armorists (do humans make large Constructs, etc.). It also made sense thematically since gnomes are known for being tinkerers. 

Tool Proficiency

When you adopt this specialization at 3rd level, you gain proficiency with smith's tools. If you already have this proficiency, you gain proficiency with one other type of artisan’s tools of your choice.

Armorist Spells

Starting at 3rd level, you always have certain spells prepared after you reach particular levels in this class, as shown in the Armorist Spells table. These spells count as artificer spells for you, but they don’t count against the number of artificer spells you prepare.

Armorist Spells

Artificer Level	Spell
3rd	Color Spray, Entangle
5th	Mirror Image, Scorching Ray
9th	Clairvoyance, Melf's Minute Meteors
13th	Fire Shield, Locate Creature
17th	Immolation, Telekinesis

Assemble Mobile Construct

At 3rd level, whenever you finish a long, or short rest, you build a construct that you can sit inside and control. You can enter or exit the Construct with a bonus action. While inside, you are considered a medium creature (see Inside and Outside the Construct sidebar), the construct counts as your Arcane Focus, and it shares your AC with an additional +1 bonus to Armor Class. Increase your AC by +2 (total) at 9th level and by +3 (total) at 15th. You can voluntarily dismiss the Construct with a bonus action and it collapses into heaps of wood, stone, and metal in its space. The Construct will also fall to pieces if you are reduced to 0 HP or less while inside it, or are targeted by effects that dispel magic (consider its "spell level" to be half your Artificer Level).

 

Sidebar: Inside and Outside the construct

While piloting the construct, increase your height by 4 feet and your weight by 200 pounds. You are considered to be one creature and it can't be targeted separately. You are not considered an object for any effects that target objects (like doing additional damage). When outside of the Construct, it is considered a medium sized object with 1 HP per artificer level and AC 15. While an object, the Construct has no Move rate, can't take actions, and will fall to pieces if reduced to 0 HP or less. It can be moved by an outside force, but otherwise just stands in its space.

Enchanted Armor Scales

Also at 3rd level, you may spend a spell slot to layer your Construct with magically-cured scales as you're building it. Gain Temporary Hit Points equal to 4 per level of the spent spell slot (so a 3rd level Spell Slot would grant you 12 temp HP).

If you have any of this Temp HP left when you dismiss the construct (a bonus action), you may also create a small explosion around yourself. Each creature other than you in a 20-foot-radius sphere centered on your location must make a Dexterity Save. Each target takes 2d8 bludgeoning damage on a failed save, or half as much damage on a successful one. This damage increases to 3d8 at 9th level, and 4d8 at 15th level.

Runic Protections

Starting at 5th level, you can mark your construct with special elemental runes whenever you finish a long or short rest. Choose one of the following damage types: acid, cold, fire, lightning, poison, psychic, or thunder. You have resistance to the selected element type until you use this feature again.

In addition to resistance, whenever a creature moves out of your melee reach, you can zap them with a bolt of elemental energy. Make a ranged spell attack. On a hit you deal damage of the selected damage type equal to 1d4 + your Intelligence Modifier. This damage increases to 2d4 + your Intelligence Modifier at 10th level, and 3d4 + your Intelligence Modifier at 15th level. You can do this a number of times equal to your Intelligence Modifier (minimum 1) and regain all uses after a long or short rest. This does not use your Reaction, but you cannot target a creature with this feature and an opportunity attack in the same round.

Elemental Wards

You can empower your elemental runes for added protection for a short while starting at 9th level. As an action, you have immunity to the chosen element type for the next ten minutes and may also grant resistance to up to four other creatures for the same amount of time. You regain this feature after you finish a long rest.

Adamantine Lamellar Plates

At 15th level, you begin to add thin plates of adamantine to your Construct while building it. You Ignore all critical attacks made against you (treat them as a regular attacks). IN addition, while this aspect of the feature is active, and you dismiss your Construct in an explosion using the Enchanted Armor Scales feature, the damage type becomes Bludgeoning and Piercing damage.

Alternatively, you can sacrifice this feature as your Reaction to reduce the total damage received from one source to zero. You regain this feature after you finish a long rest.