This primarily focuses on a firearm being your weapon, either a Pepperbox or Bad News or your own creation - at the discretion of your DM - and utilizing spells that benefit allies to synergize and empower tactics.

Tool Proficiency

3rd-level Artillerist feature

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.

Arcanoneer Spells

3rd-level Arcanoneer feature

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

Artillerist Spells

Artificer Level	Spell
3rd	magic missile, faerie fire
5th	enhance ability, cordon of arrows
9th	elemental weapon, lightning arrow
13th	storm sphere, arcane eye
17th	cone of cold, swift quiver

Leyline Inscription

3rd-level Artillerist feature

You have learned to infuse your firearm with the arcane focus needed to cast spells. Using smith's tools, you can take 1 hour to take a useable firearm and give it the arcane leylines to cast spells. This firearm must be wieldable one- or two-handed (i.e. no giving ship cannons arcane leylines), it still uses ammunition as normal, unless you use a spell that facilitates magical ammo.

Once you inscribe the leylines, you can’t do so again until you finish a long rest. You can inscribe leylines onto a new gun, which difuses the leylines on your previous firearm.

The firearm is considered a magical object.

When you inscribe the leylines on your firearm, you can choose one of these leylines to enhance your firearm. You can change these leylines by using 1 hour to inscribe new ones.

Leyline Inscriptions

LEYLINE	EFFECTS
Arcanum	When making ranged attacks with this weapon, you can use your Intelligence modifier instead of your Dexterity modifier. When you cast a 1st-level or higher spell using this inscribed weapon as a focus, you can give an ally within 15ft of you Temporary HP equal to half your Artificer level (rounded down). This ability can be activated a number of times equal to your proficiency bonus and refreshes after a short rest.
Chaos	When you roll damage on a ranged attack with this inscribed weapon, you can add your Intelligence modifier to your damage. When this firearm backfires, it causes 1d8 force damage to all creatures within 10ft of you. When you roll a natural 20 on a ranged attack with this firearm, you cause 1d8 force damage to all creatures within 10ft of the target. When you are struck by a 1st-level or higher spell with elemental damage, you can use your reaction to receive the elemental force and channel it into your leylines, changing from Piercing damage to the new elemental type. You can do this a number of times equal to your Intellect modifier, resets after a short rest.
Dragonic	The misfire score for this firearm increases by 1, add 1d4 fire damage to its attacks. As a bonus action you can charge this firearm with Explosive ammo, regardless of the ammo currently loaded. The next shot from this firearm is considered Explosive, afterward raise the Misfire score by 1. The misfire score will reset after you take 1 hour with the Smith's Tools to fix it (the cost of which is 1g X number of misfires removed). [Explosive:  Upon a hit, everything within 5 ft of the target must make a Dexterity saving throw (DC equal to 8 + your proficiency bonus + your Intelligence modifier) or suffer 1d8 fire damage. If the weapon misses, the ammunition fails to detonate, or bounces away harmlessly before doing so.]

Extra Attack

5th-level Arcanoneer feature

You can attack twice, rather than once, whenever you take the Attack action on your turn.

Arcane Ammunition

9th-level Arcanoneer feature

During a short rest, you can infuse a number of ammunition you have with spells you currently have prepared. You can infuse a number of level spells equal to half your Artificer level, rounded down (i.e. at level 9, you can infuse 4 levels worth of spells into ammunition, so four 1st-level spells, two 2nd-level spells, a 3rd- and 1st-level spell, etc.). These ammunition retain the spell for a number of days equal to your Intelligence modifier (minimum 1), also when the ammunition is used, you do not need the spell prepared to activate the ammunition. 

If you infuse more ammunition during a different short rest, the previous ammunition will lose their magic.

When you use the spell-infused ammunition, it will cast as normal, but will raise the misfire score of your firearm by 1.

Also gain the following Leyline Inscription:

LEYLINE

EFFECT

Farseeker

You can fire your firearm the max distance without suffering from disadvantage, also when enemies are hit by this weapon's attack, they are marked. While marked, spells cast at 1st-level or higher toward the target are cast at advantage. You can only mark one enemy with Farseeker's ability, and when the marked enemy is hit by your spell, the mark fades. You can mark enemies a number of times equal to your Intelligence modifier (min 1), and regain uses after a short rest.

Leyline Mastery

15th-level Arcanoneer feature

You can now have two Leyline inscriptions active - they can be on the same firearm or separate ones - also, you can inscribe Leylines after a short rest instead of a long rest.

If your Leyline-inscripted firearm is attuned to you, it does not take up an attunement slot.

LEYLINE

EFFECT

Union

When using this firearm as a focus for casting a 1st-level or higher spell and this spell has a range other than 'self' or 'melee', you can instead use the range of the firearm as the casting range. Each time you cast a 1st-level or higher spell with this firearm as the focus, this firearm gains a charge, and it can hold no more than three charges. When you make a ranged attack with this firearm, you can expend a charge to add a d4 to the attack roll - not damage.