Path of the Slayer

 Barbarians which follow the Path of the Slayer represent a more technologically advanced sect of the typically primal warriors. They freely employ their strength to expand their capacity in using the most unwieldy of modern firearms, and their primitively strong wills allow them to make bold plays beyond the daring of more civilized gunslingers.

A Barbarian which follows the Path of the Slayer might be a brutish mercenary wielding heavy weapons, a devout crusader for a cause willing to defy typical traditions, or even a mindless automaton unstoppable to perform its mission.

Greatarms

Starting 3rd level, you have the means to commission bulky, custom firearm modifications off-quest to turn firearms into your greatarms. You are proficient with firearms.
Modifying a firearm into a greatarm costs parts and labour, a GP price equal to a fifth of the minimum price of the firearm, to a maximum price of 25000 GP. Greatarms use custom ammo costing 2 GP each, and weigh thrice as much as the original firearm. Creatures other than you automatically misfire unless on a roll of 20 if they attempt to attack with your greatarm, as it is simply too awkward and cumbersome.
When you commission a greatarm, you can choose a number of modifications for it up to half your proficiency bonus (Detailed at the end of this document).

You can have up to 1 greatarm at any time. 2 at 13th level and 3 at 17th level. A greatarm can be restored to the original firearm by any creature over 8 hours of work.

Brutish Gunslinger

At 3rd level, you can meet the call of sheer strength your custom greatarms require to control. You can use your strength score and modifier in place of dexterity for attacks made with greatarms within 30 feet. Ranged attacks made with greatarms in this range can benefit and count as melee and melee weapon attacks for barbarian class features and boons.

CQC

Starting at 6th level, you're an expert in close range shooting. While raging, you do not have disadvantage on ranged attacks within 5 feet of you. As a reaction when a melee attack is rolled against you, you can make a targetless attack with a firearm. If the attack roll of your attack is greater, the melee attack misses, and your attack then targets the original attacker. You add your proficiency bonus again to hit for attacks made with this feature.

Percussive Maintenance

At 6th level, you have an uncanny knack of knocking things into line. Once per short rest, when you make a tools check and fail, you can add half your strength modifier (rounded up) to the check, potentially turning it into a success. This feature can also be used on charisma checks made within melee range of you, possibly changing the result at DM's discretion.

You can expend 2 hit die to reload a firearm as a bonus action.

Bellowing Presence

At 10th level, your primal reactions are honed to an edge, and you have the daring to make selfless decisions in a split second. When you make an attack, you can chose to antagonize the target. When an antagonized creature targets a friendly creature within 10 feet of you with a harmful effect, you can expend 2 hit die to become the new target of the effect. If the effect targeted you as well, you sustain it twice.

Your CQC feature now functions against ranged attacks made in range of Brutish Gunslinger, and can be used when a creature within 10 feet of you is targeted by an attack as well.
A target stops being antagonized at the end of your next turn.

Tactical Intuition

At 14th level, you are a relentless embodiment of primal rage and a knowledgeable sommelier of your lethal tools. While raging, you know the location of antagonized creatures, and you can use CQC without a reaction by expending a hit die. You can reload a greatarm you are holding when you reduce a hostile creature to 0 hitpoints, and you can expend 4 hit die to attempt to fix a misfired firearm as a bonus action.

You can use CQC by expending 2 hit die when you or a creature within 10 feet is targeted by a harmful effect which includes a saving throw. If your attack roll is greater than the DC of the saving throw, the target gains a bonus to their save equal to your strength modifier, and CQC continues as if you had beaten the attack roll of the creature who caused the effect.

You can identify the properties, base damage, and features, magical or mundane, of any weapon you see.

Greatarm Modifications:

The following modifications are available for a greatarm, and change how it operates. Modifications of the same type are mutually exclusive.
Greatarm DC: DC(8 + Strength Modifier + Proficiency Bonus + Weapon Bonus)

Type A	Extreme Caliber: You deal double damage to objects and structures. When you hit a large or smaller target, you can push it up to 10 feet away from you. Each attack you make on a turn after the first takes a -2 penalty to hit.	Tether Bolt: When you hit a creature with this firearm, their movement away from you is considered difficult terrain until the end of your next turn. You can only attack one target with this firearm each turn.	Energy: The base damage type of this firearm becomes fire, lightning, radiant or acid, chosen upon creation.
Type B	Full Auto: Once per turn, you can expend 3 pieces of ammunition on an attack. When you do so, choose up to 2 targets within 10 feet of the original. They must make a dexterity saving throw. On a fail, they take the damage of the original attack (rerolled each).	Explosive: When you make an attack and hit, creatures and objects besides the target within 5 feet of it must make a dexterity saving throw. On a fail, they take the weapon’s base damage plus the weapon bonus.	Strong Rifling: The maximum range at which your brutish gunslinger feature functions is doubled.
Type C	Chain Fed: The reload score of this firearm is doubled, to a minimum of 6. If it does not have one, it gains one (6) and loses the loading property. Reloading this firearm takes a minute. This time cannot be reduced.	Charge: You can forgo an attack on your turn. On the next attack you make with the firearm until the end of your next turn, double your proficiency bonus to hit, ignore temporary bonuses to AC, and any reactions made on the attack.	Tacticool: When you use reckless attack, you gain an additional 5 feet of movement until the end of your turn.