Base Class: Fighter
You have spent time studying the explosive art of gunpowder and its many uses.
Basics of Design
Upon choosing this archetype at 3rd level, you gain proficiency with Tinker’s Tools. Given you have resources on hand, you may use them to craft ammunition at half the cost, repair damaged firearms, modify existing firearms, or even draft and create new ones (DM’s discretion).
There are various upgrades one can make to a firearm. Each has a Gun-Part Category (GPC), Time, and Cost associated with it. A single gun can have no more than 3 upgrades associated with it. Upgrades for all GPC categories do not stack, aside from the Chamber category. Removing an upgrade is easy enough, but the act destroys the upgraded piece (ex. you may remove Modified Sights to equip a Short Scope, but those sights are destroyed in the process.
The time associated with each upgrade assumes you are spending at least 4 hours a day working on its construction. You may work on no more than 2 upgrades at a time.
Although there are a set of standard upgrades associated with firearms, a skilled gunsmith can even attempt to create their own custom designs (DM's Discretion). The following are the standard upgrades (divided by GPC) one could apply to a gun:
GPC - Sights:
Modified Sights. Time: 2 days. Cost: 60 Gold.
Effect: +1 to hit.
Short Scope. Time: 5 days. Cost: 100 Gold.
Effect: +2 to hit target further than 60 feet away. Disadvantage to hit <60 feet.
Long Scope. Time: 1 week. Cost: 300 Gold
Effect: Weapon ignores disadvantage at ranges exceeding its normal range. Any attack made below its normal range is rolled with disadvantage.
GPC - Barrel:
Short Barrel. Time: 1 day. Cost: 30 Gold.
Effect: Normal and Max ranges are reduced by half. Ignores disadvantage imposed when firing in close quarters. -2 to damage rolls.
Long Barrel. Time 5 days. Costs 80 Gold.
Effect: Normal range is increased by 60 feet. +2 to damage rolls.
Bayonet. Time: 1 Day. Cost 15 Gold.
Effect: Grants ability to Bonus Action Melee attack (Dagger stats). Only available on rifles.
GPC - Chamber:
Increased Capacity. Time: 1 days/round. Cost: 100 Gold/round. Maximum of +4 Round Capacity.
Effect: Ammo capacity increases up to +4.
Semi-Auto Chamber. Time: 1 week. Cost: 500 Gold.
Effect: Gun may be fired twice (given it has a capacity more than 1) per attack.
Speed Loader. Time: 5 days. Cost: 200 gold.
Effect: May reload gun as a Bonus Action.
GPC - Stock/Grip:
Enhanced Stock/Grip. Time: 2 days. Cost: 60 Gold.
Effect: +1 to hit.
Bombardier
Starting at 3rd level, you gain the ability to craft small bombs. Only you know the intricacies of these volatile weapons, and therefor only you can wield them. Attacks with bombs count as a ranged attack (with proficiency), but go against a set DC rather than a creature's armor class (with exception to the Molotov). The DC varies depending on the bomb of choice.
Due to the volatility of these weapons, the chance for mishaps is greater than normal. If you roll a 1 or 2 on your attack roll, the bomb detonates early, effecting yourself and those within the radius of the bomb's blast. With volatility also comes quick expiration dates. Unused bombs are rendered inert after a long rest.
A bomb requires 5 gold pieces worth of materials to create. You may craft a number of bombs equal to half your Gunsmith level (rounded down) per long rest and may choose from the following options, or work with your DM to try and create your own:
Frag: Atk DC: 15
This bomb sends shrapnel soaring throughout an area. Range: 30/60, any creature within a 10-ft radius must succeed on a Dexterity Saving Throw equal to 8+ your Intelligence modifier or take 2d10 piercing damage, half on a success.
Flash: Atk DC: 14
This bomb explodes with a blinding flash of light. Range: 30/60, any creature within a 10-ft radius must succeed on a Constitution Saving Throw equal to 8+ your Intelligence modifier or become blinded until the start of your next turn.
Cracker: Atk DC: 14
This bomb bursts in a harmless, but thunderous manner. Range: 30/90, any creature within a 10-ft radius must succeed on a Constitution Saving Throw equal to 8+ your Intelligence modifier or become distracted. On a fail, the next attack agains them is made with advantage.
Molotov: No Atk DC, goes against enemy's armor class.
This bomb bursts into a raging inferno. Range: 20/60, anything flammable within a 5-foot cube instantly ignites. A creature hit with the molotov takes 2d6 fire damage and must use an action to extinguish the flames, or take an additional d6 fire damage at the end of their turn.
Gun Smarts
Starting at 7th level, you may add your intelligence modifier to ranged attack roles made with a firearm.
Extract Details
Starting at 10th level, you are able to spot the most minute of elements a firearm possesses. You can determine the qualities (including damage dice, bonuses to hit, bonuses to damage, effective range, etc.) of any gun you can see within 60 feet. It requires no action on your part to assess the guns in your surrounding and shout out the details to your teammates.
Jammer
Starting at 15th level, you may make a Grapple attack against an enemy wielding a firearm as a Bonus Action. On a successful grapple, you jam the enemies gun.
Preternatural Defense
Starting at 18th level, your expertise in firearms allows you to anticipate the moment before being shot, giving you the smallest window to avoid deadly hits. You become resistant to piercing damage from firearms.