This is a variation on Mr. Mercer's Gunslinger subclass.

Most warriors and combat specialists spend their years perfecting the classic arts of swordplay, archery, or pole arm tactics. Whether duelist or infantry, martial weapons were seemingly perfected long ago, and the true challenge is to master them.
However, some minds couldn’t stop with the innovation of the crossbow. Experimentation with alchemical components and rare metals have unlocked the secrets of controlled explosive force. The few who survive these trials of ingenuity may become the first to create, and deftly wield, the first firearms.
This archetype focuses on the ability to design, craft, and utilize powerful, yet dangerous ranged weapons. Through creative innovation and immaculate aim, you become a distant force of death on the battlefield. However, not being a perfect science, firearms carry an inherent instability that can occasionally leave you without a functional means of attack. This is the danger of new, untested technologies in a world where the arcane energies that rule the elements are ever present.
Should this path of powder, fire, and metal call to you, keep your wits about you, hold on to your convictions as a fighter, and let skill meet luck to guide your bullets to strike true.

Level 3: Firearm Specialist

You ignore the Loading property of firearms and you gain proficiency with Tinker’s Tools. You may use them to craft ammunition at half the cost, repair damaged firearms, draft upgrades or create new firearms (DM’s discretion). Some extremely experimental and intricate firearms are only available through crafting.

Firearm Properties

Firearms are a new and volatile technology, and as such bring their own unique set of weapon properties. Some properties are followed by a number, and this number signifies an element of that property (outlined below). These properties replace the ones presented in the Player’s Handbook. Firearms are ranged weapons.

Ammunition. All firearms require ammunition to make an attack, and due to their rare nature, ammunition may be near impossible to find or purchase. However, if materials are gathered, you can craft ammunition yourself using your Tinker’s Tools at half the cost. Each firearm uses its own unique ammunition and is generally sold or crafted in batches listed below next to the price.

Loading. Because of the time required to load this weapon, you can fire only one piece of ammunition from it when you use an Action, Bonus Action, or Reaction to fire it, regardless of the number of attacks you can normally make.

Loud. After being shot, the firearm emits a thunderous boom audible out to five times the weapon's long Range.

Misfire. Whenever you make an attack roll with a firearm, and the dice roll is equal to or lower than the weapon’s Misfire score, the weapon misfires. The attack misses, and the weapon cannot be used again until you spend an action to try and repair it. To repair your firearm, you must make a successful Tinker’s Tools check (DC equal to 8 + misfire score). If your check fails, the weapon is broken and must be mended out of combat at a quarter of the cost of the firearm. Unmodified weapons have a misfire score of 0, and each upgrade made in a firearm may increase its misfire score. Creatures who use a firearm without being proficient with them increase the weapon’s misfire score by 1.

Reload. A limited number of shots can be made with a weapon that has the Reload property. A character must then reload it using an action or a bonus action (the character's choice).

Firearms

Name	Damage	Properties	Mastery	Weight	Cost
Pistol	1d10 Piercing	Ammunition (Range 30/90; Bullet), Loading	Vex	3 lb.	250 GP
Musket	1d12 Piercing	Ammunition (Range 40/120; Bullet), Loading, Two-Handed	Slow	10 lb.	500 GP
Blunderbuss	4d6 Piercing	Ammunition, Heavy, (25-foot-long, 5-foot-wide Line*; Bullet), Reload (1 shot), Two-Handed	Push	20 lb.	Crafted

*Each creature in the Line that isn’t behind half cover or more must make a DC 15  Dexterity saving throw, taking 4d6 piercing damage on a failed save, or half as much damage on a successful one. Each shot uses 4 bullets.

Firearm Upgrades

The following list provides various options to enhance your firearm’s capabilities (options that increase the damage of the firearm can't be stacked unless stated on the option).

Crosshairs. (Prerequisite: None) You equip your firearm with a targeting mechanism. If you haven't moved this turn, you can aim down your sights as a Bonus Action, reducing your speed to 0 and granting you Advantage on all attacks you make using this firearm until the end of your turn.

Enhanced Bullets. (Prerequisite: Pistol or Musket) You design better bullets for your firearms. Upon a hit, you gain a +1 bonus to damage rolls you make with that weapon. You can enhance your weapon this way up to 3 times, and each time after the first increases the misfire score by 1.

Incendiary Rounds. (Prerequisite: Musket or Slug Shot) You design an explosive bullet for your firearms. Upon a hit, the weapon deals its normal damage and every creature within 10-foot radius of the target must make a Dexterity saving throw (uses your Trick Shot save DC) taking 2d4 fire damage on a failed save, or half as much damage on a successful one. If the shot misses, the ammunition fails to detonate and bounces away harmlessly. Increases the misfire score by 2.

Rifling. (Prerequisite: Pistol or Musket) You design spiral grooves inside the barrel of your firearm, increasing its accuracy. You gain a +1 bonus to attack rolls you make with that weapon. You can enhance your weapon this way up to 3 times, and each time after the first increases the misfire score by 1.

Slug Shot. (Prerequisite: Blunderbuss) You design a special bullet for a blunderbuss that transforms its properties. When loaded, the weapon fires at a single target, deals 2d6 + your Dexterity modifier piercing damage, and have the following properties: Ammunition, Heavy, (25/75; Bullet), Reload (2 shots), Two-Handed

Suppressor. (Prerequisite: Pistol or Musket) You design a device that suppresses the blast created by your firearms when they are discharged. Attaching a suppressor to your firearms causes noise emitted to be audible only to the weapon's normal Range. It can be attached or detached from your firearm with a Bonus Action. When attached, it removes any bonus damage the weapon has.

Level 3: Adept Gunner

You learn to perform powerful trick shots to disable or damage your opponents using your firearms.

Trick Shots. You learn three trick shots of your choice, which are detailed under “Trick Shots” below. Many trick shots enhance an attack in some way. Each use of a trick shot must be declared before the attack roll is made. You can use only one trick shot per attack. You learn an additional trick shot of your choice when you reach Fighter levels 7, 10, and 15. Each time you learn new trick shots, you can also replace one maneuver you know with a different one.

Grit. You have grit points equal to your Dexterity modifier. You regain 1 expended grit point each time you roll a Critical Hit on the D20 Test for an attack with a firearm, or deal a killing blow with a firearm to a creature. You regain all expended grit points after a Short or Long Rest. 

Saving Throws. Some of your trick shots require your targets to make a saving throw to resist the trick shot’s effects. The saving throw DC is calculated as follows:

   Trick Shot save DC = 8 + your proficiency bonus + your Dexterity modifier

Level 7: Sharp Reflexes

You learn how to quickly handle your guns. You can add your Proficiency Bonus to your Initiative. Additionally, you can attempt to repair a misfired (but not broken) firearm as a Bonus Action.

Level 10: Vicious Critical

Your firearm attacks score a Critical Hit on a roll of 19-20. Additionally, whenever you score a Critical Hit with your firearms, the target suffers extra damage equal to your level.

Level 15: True Grit

Once per turn you can use a Trick Shot without spending a grit point.

Level 18: Crack Shot

You have mastered the use of your trick shots. You can now use two trick shots per attack.

Trick Shots

These trick shots are presented in alphabetical order.

Bullying Shot

You can use the powerful blast and thundering sound of your firearm to shake the resolve of a creature. You can expend one grit point while making a Charisma (Intimidation) check to gain advantage on the roll.

Dazing Shot

When you make a firearm attack against a creature, you can expend one grit point to attempt to dizzy your opponent. On a hit, the creature suffers normal damage and must make a Constitution saving throw or suffer Disadvantage on attacks until the end of their next turn.

Deadeye Shot

When you make a firearm attack against a creature, you can expend one grit point to gain Advantage on the attack roll.

Disarming Shot

When you make a firearm attack against a creature, you can expend one grit point to attempt to shoot an object from their hands. On a hit, the creature suffers normal damage and must succeed on a Strength saving throw or drop 1 held object of your choice and have that object be pushed 10 feet away from you.

Distracting Shot

When you make a firearm attack against a creature, you can expend one grit point to distract the creature, giving your allies an opening. The next attack roll against the target by an attacker other than you has Advantage if the attack is made before the start of your next turn.

Forceful Shot

When you make a firearm attack against a creature, you can expend one grit point to attempt to force them back. On a hit, the creature suffers normal damage and must succeed on a Strength saving throw or be pushed 15 feet away from you.

Goading Shot

When you make a firearm attack against a creature, you can expend one grit point to attempt to goad the target into attacking you. The target must make a Wisdom saving throw. On a failed save, the target has Disadvantage on all attack rolls against targets other than you until the end of your next turn.

Maneuvering Shot

When you make a firearm attack against a creature, you can expend one grit point to maneuver one of your comrades into a more advantageous position. You choose a friendly creature who can see or hear you. That creature can use its reaction to move up to half its speed without provoking Opportunity Attacks from the target of your attack.

Piercing Shot

When you make a firearm attack against a creature, you can expend one grit point to attempt to fire through multiple opponents. The initial attack gains a +1 to the firearm’s misfire score. On a hit, the creature suffers normal damage and you make an attack roll with Disadvantage against every creature in a line directly behind the target within your first range increment. Only the initial attack can misfire.

Violent Shot

When you make a firearm attack against a creature, you can expend one or more grit points to enhance the volatility of the attack. For each grit point expended, the attack gains a +2 to the firearm’s misfire score. If the attack hits, you can roll one additional weapon damage die per grit point spent when determining the damage.

Winging Shot

When you make a firearm attack against a creature, you can expend one grit point to attempt to topple a moving target. On a hit, the creature suffers normal damage and must make a Strength saving throw or be knocked prone.