Gunsmith

From pistols to anti-aircraft cannons, the gunsmith is at the forefront of firearms. Since the dawn of time, man has created tools to farm, fight, and defend. Attacking from afar has always granted an advantage over fighting your enemy face to face. The evolution of ranged weaponry goes from the sling, to the bow, to the crossbow, and to the first rudimentary gun which was a firework launcher. Over time, inventors twisted metal and miniaturized the cannon to fire round bullets. From there, the Americans were some of the first to invent barrel rifling which made the musket more accurate by spinning the projectile whilst still inside the barrel so it wasn't as burdened by friction. Bullets went from round to long and pencil-like and the weapons in which they were fired from evolved from being mostly wood, to metal, to plastic. 

A gunsmith in your world could be anything from a tinkerer like Paul Revere creating weapons to defend himself from tyranny to a scrapper making guns for rations in your post-apocalyptic world. The gunsmith, however, also know how to use their weapons. Adept gunsmiths can pull off gunshots that seem impossible to the average person or devastate the enemy from behind cover. 

The design of your character's weapons are only limited by your imagination. Your rifle could be lever-action and put out several shots in a turn or in favor of accuracy, bolt action with only a single shot a turn with longer range, and a better chance to hit.

Modes of Fire

Starting at 3th level, you learn to tinker with weapons to fit any situation. You gain proficiency in tinker's tools and firearms and you learn how to create firearms and ammunition. When crafting ammunition or firearms, it only requires half of the cost of materials. You gain the ability to modify weapons with the Ammunition (Firearms) property to give it one of the following properties:

• Scattergun: A weapon that has the scattergun property holds 2 pieces of ammunition and requires an action to reload. You can make a normal single-target attack, or it can blast creatures within a 20 ft. cone of the attack must make a Dexterity saving throw with a DC equal to 8 + your Dexterity modifier + your proficiency bonus or take damage equal to your Sneak Attack damage. This action uses 2 pieces of ammunition.
• Rapid-Fire: A weapon that has the rapid-fire property holds 20 pieces of ammunition and requires an action to reload. In addition, you can make a normal single-target attack, or it can spray a 10-foot-cube area within normal range with shots. Each creature in the area must succeed on a Dexterity saving throw equal to 8 + your Dexterity modifier + your proficiency bonus or take damage equal to your Sneak Attack damage. This action uses ten pieces of ammunition. 
• Precision: A weapon that has the precision property holds 1 piece of ammunition and requires a bonus action to reload. In addition, you can make a normal single-target attack or, if you moved less than half of your movement speed this turn, you may cause the shot to potentially go through one target and hit another. The shot travels in a straight line up to 240 ft. or until it hits deals no damage or hits a solid surface. Creatures in this line must make a Dexterity saving throw equal to 8 + your Dexterity modifier + your proficiency bonus. On a failure, the first creature suffers damage equal to your Sneak Attack damage. Each creature that fails beyond the first takes damage equal to your Sneak Attack damage with one die subtracted for each target already hit.
• Pistoleer: You may only give this property to a weapon with the Ammunition (Firearms) property without the two-handed property. It holds 6 pieces of ammunition that require a bonus action to reload. You may make an additional attack as a bonus action. When wielding two weapons with this property, you may make two attacks (one with each weapon) as a part of the same action. If you fire both of the attacks at the same target, they suffer Sneak Attack damage in addition to any other damage they take.

Modifying a weapon takes 1 minute and a weapon cannot have more than one of the properties listed. You can modify a weapon a number of times equal to your proficiency bonus before finishing a long rest.

Precise

When you choose this archetype at 3rd level, you gain proficiency in the Perception and Insight skills if you don’t already have it. Your proficiency bonus is doubled for any ability check you make that uses either of those proficiencies.

Gunslinger

At 9th level, you have become an exceptionally swift and accurate gunman. Attacks made with firearms ignore resistance to piercing damage. In addition, your attacks ignore all cover except full-cover and you may draw and attack with a firearm as a part of the same action.

Supporting Fire

Starting at 13th level, you've learned how to distract enemies and adapt your weapons faster. As a reaction to a creature making an attack against an ally, you may make a ranged weapon attack against that creature in addition to giving an ally a bonus to their AC equal to your Dexterity modifier until the end of the turn. In addition, you may use a bonus action instead of taking a minute when using the feature of Modes of Fire. You may use this feature a number of times equal to your proficiency bonus. 

Master of Arms

Starting at 17th level, your knowledge of firearms has reached its zenith. The properties from the Modes of Fire feature gain the following in addition to their normal properties:

• Scattergun: When using the alternative fire, the cone size is increased to 30 ft. and only requires a bonus action reload. Targets that fail the saving throw suffer half of the damage dealt at the start of their next turn.
• Rapid-Fire: When using the alternative fire, the cube size is increased to 15 ft. and only requires a bonus action to reload. Targets within the cube gain no benefits to cover unless they are in total cover.
• Precision: Creatures with immunity to piercing damage that suffer damage from this weapon are treated as though they only had resistance to piercing damage. In addition, attacking from long range doesn't impose disadvantage on attack rolls. 
• Pistoleer: Attacking within engagement range doesn't impose disadvantage on your attack rolls. In addition, when making a weapon attack against a target within 5 ft. of you, attacks made with a weapon with this property score of a critical hit on a roll of 18–20.