Credit for this subclass goes to DnDDangan as this is his Order of the Arcane Arsenal but modifed for an artificer.

Arcane Arsenal

Beginning at 3rd level you gain acsess to your arcane weaponry arsenal, that grants you certain benefits. You can choose two weapons at lvl 3, and one additional one at lvl 5, lvl 9th,lvl 11 and lvl 15. While wielding your arcane weapons, you are classed as proficient with all of them. You can not infuse these weapons as they already magical.

Switching. As a Bonus Action, you can shift the form of your arcane arsenal to suit your needs better. You can change your weapons a number of times equal to double your proficiency bonus. You regain all expended switches after finishing a long rest

Additonally at lvl 3 you gain proficiencies with Smith's Tool's if you do not already.

Arkal and Dormus

This arcane shotgun and shield are a constant pair of two. They give you the ability to stand in front of the fight, blocking incoming damage with arkal and hitting hard with dormus.
Arkal (Shield) counts as a +1 Shield with a base AC of +2.
Dormus (Shotgun) counts as a +1 Shotgun. Dormus deals 2d8 Piercing damage, has a Range of a 20 feet and has to reload after two shots.

Upgrade:
After switching to Arkal and Dormus, you get a +2 Bonus to all damage rolls you make until the start of your next turn.

While wielding Arkal and Dormus, you can use an Action to combine the two. After doing so, you lose the bonus to ac, but gain the following benefits for 1 minute or until you switch to another weapon:
Dormus deals 3d8 Piercing damage
Dormus gains the Two-Handed Property and grows in size

After doing this, Arkal and Dormus go dormant for the day and cant be used again until you finish a long rest.

Gilgathon

This arcane quarterstaff gives you the ability to hit your enemies from a distance. Gilgathon counts as a +1 quarterstaff for all attack and damage rolls. Gilgathon has a reach of 10 Feet. While wielding Gilgathon, as a bonus action, you can extend the reach of the weapon by an additional 5ft.
Upgrade:
After you switch to Gilgathon, you can activate Gilgathons second mode which reinforces the edges of the staff with a dense metal. Your damage die become a d8 and a d10 when using the versatile property.

Once per day, you can extend the reach of Gilgathon to 25 feet for one round.

Jarrithas

This arcane rifle is the perfect weapon for staying out of combat and attacking from a long distance.

Jarrithas counts as a +1 Sniper Rifle for all attack and damage rolls made with it.
Jarrithas has a range of 200/800ft, has the two-handed property, has to be reloaded after each shot and deals 3d12 piercing damage.  
While wielding Jarrithas, you can only attack once per turn, regardles of how many extra attacks you would normally get.

Upgrade:
After switching to Jarrithas, you can make your next attack with disadvantage but deal an additional 1d12 damage if it hits.
Once per day, Jarrithas can fire a massive bullet that explodes on impact. As an attack, you can fire this bullet, to deal 4d12 piercing damage in a 20 foot radius. Each creature standing in the radius has to make a dexterity saving throw against your spell save dc. On a fail, they take the full damage. On a success they take half the damage.

Maroch

This ancient longbow has been carfted to make its own arcane arrows allowing for multiple shots to be fired.

Maroch counts as a +1 Longbow for all attack and damage rolls you make with it.
Maroch doesnt use ammunition and does not have to be reloaded.
Maroch has a range of 150/600ft
Maroch fires a second shot immediatly after the first one if you score a critical hit.
While wielding Maroch, you gain one additional attack.
You have advantage on any attack role you make against an undead.

Upgrade:
After switching to Maroch, your first shot you make with it counts as two.
Once per day, you can use the energy within Maroch to fire a rapid series of arcane arrows at your enemies. After using this as a bonus action, you fire 3 arrows for every attack for 1 round.

Serillin

This magically enhanced gun can channel the energy within you to fire powerful bursts of magic. The weapon is specialised in short range fights against monstrosities.

Serillin has a range of 60 feet.
Serillin counts as a +1 Weapon for Attacks and damage rolls
It deals 2d6 Piercing damage and uses your dexterity modifier for attack and damage rolls.
Serillin has to be reloaded after 12 shots.

Upgrade:
After switching to Serillin, every attack you make in the next round is made with advantage.
Additionally you can fire a stronger bullet of arcane force, dealing 2d6 additional force damage.
This damage increases by 1d6 if you hit a monstrosity with it. After firing this bullet, you always have to reload serillin. You can fire this bullet 3 times per day.

Tekunoth

This magically enhanced dagger gives you the ability to quickly move around the battlefield, delivering many strikes and cutting your foes.

Tekunoth counts as a +1 dagger.
While wielding Tekunoth, you can use the dash and the disengage action as a bonus action.
While wielding Tekunoth, your movement speed increases by 10 feet.

Upgrade:
After switching to Tekunoth, each creature in a 5 feet radius around you takes 1d6 slashing damage.
Additionally, you can unleash a series of attacks on every enemy in your area. Once per day, you can use one of your attacks to attack every creature in a 5 feet radius around you. For each creature, you have to roll one attack roll. If you hit, you deal 2d6 slashing damage.

Extra Attack

Beginning at 5th level, you can attack twice, instead of once, whenever you take the Attack action on your turn.

Upgrade

Starting at 9th level, you've spent enough time fighting with your weapons to notice what can be improved. You can now upgrade two of your weapons and use their additional features.

Perfected Arsenal

At 15th level, you can upgrade all of your current arcane arsenal.

Additionally, you learn the last of your Arcane Weapons and can now use all of them.

Additionally you can upgrade 2 more of your arcane weapons and use their additonal features.