Bandle Gunner

Blast away at swarms of enemies, watching them disappear as you put explosive charges on them. Hit them with the timer, then rocket jump away before the explosion.

Never look back, because cool people don't look at explosions.

Gun Proficiency

Beginning when you choose this archetype at level 3 you gain proficiency with massive guns. You ignore size restrictions when using these weapons. You also gain proficiency in tinkers tools.

Your gunner abilities are explosive and require your target to make a saving throw to resist their effects. The saving throw DC is calculated as follows:

Explosion save DC = 8 + proficiency bonus + your Dexterity modifier

Explosive Shots

Also starting at level 3 when you land the killing blow on a hostile creature wielding a massive gun you may have it die in an explosive way. Creatures within 5ft. must make a constitution saving throw or take 1d8 thunder damage. This thunder damage increases to 2d8 at level 7, 3d8 at level 10, 4d8 at level 15, and 5d8 at level 18.

Rocket Jump

Starting at level 7, as a bonus action, you can jump up to 30 feet and lands with a blast. Everyone in a 10 feet of the impact needs to make a strength save against your explosion save DC. On a failed save, a creature takes 2d4 fire damage and is knocked prone. On a successful save, it takes half as much damage and aren't knocked prone.
If you land directly on top of another creature, they make the saving throw with disadvantage but you'll receive the damage as well.
This movement doesn't provoke attacks of opportunity. After the jump, you move to the closest open space.
You can use this feature once per short and/or long rest.

Explosive Charge

Beginning at level 10, when you use the action surge feature you may choose a creature within 30 feet that you mark with an explosive charge. The charge explodes; at the end of your next turn, when the target dies, or when it gets hit by 5 of your attacks. 

All creatures within a 10ft. radius of the marked target must succeed on a constitution saving throw or take 2d4 thunder damage, on a successful save it takes half as much damage. The marked target makes the save with disadvantage if they are alive.

The damage and radius change depending on how many attacks with a massive gun the marked target takes.

• 1 hit: The explosive charge deals an additional 3d4 thunder damage for a total 5d4
  
• 2 hits: The radius increases to 15 feet
• 3 hits: The damage dice for all thunder damage are now d6s instead of d4s
• 4 hits: The damage dice for the thunder damage now is d8s instead of d6s and the radius is 20 ft.
• 5 hits: The damage is maximized to 40 thunder damage and you regain the use of rocket jump feature as if you had taken a short rest.

Buster Shot

Once you've reached level 15 you start to grasp the true power of your weapon. As an action you may blast a creature that you can see within 30 feet of you. The target and all creatures in a 30 foot cone behind the target must succeed on a Strength saving throw or take 10d6 bludgeoning damage and get knocked back up to 30 feet. On a successful save, it takes half as much damage and isn't knocked back.

If this ability hits a target who is marked by your explosive charge feature it counts as 2 hits for purposes of determining damage and effects.

Once you use this feature, you must finish a short or long rest before you can use it again.

Demolition Expert

By level 18 you have learned to be careful with explosions and are now immune to your own explosive abilities.

When you roll for initiative and do not have a charge of action surge, you regain one charge. You may only regain an action surge this way once per short or long rest.