Blast & Slash

Weapon (battleaxe), uncommon (requires attunement)
  A rudimentary firearm, this item has been modified and mounted with an axe head fashioned from scrap metal. Remnants of oak wood on the handle and the firing mechanism are all that is left from the original firearm. The result is a magical axe-blunderbuss hybrid that can only be used at point blank range due to the poor precision and limited blast power.
  You gain a +1 bonus to attack and damage rolls made with this magic weapon.
  Point Blank Blast. When you hit a creature with this weapon, you can activate the backfiring mechanism and deal an additional 4d6 piercing damage to the creature. The creature must succeed on a DC 15 Constitution saving throw or suffer severe burns for 1 minute. At the start of each of its turns, the creature must repeat the saving throw, taking 1d6 fire damage on a failed save. On a successful save, the effect ends. Once you use this effect, you can’t do so again until you rearrange the mechanism over a short rest.
  Very rare variant: Increase the bonus to + 2, the piercing damage to 8d6, and the fire damage to 2d6.