Genie's got a Gun

Background

The year is 301 PS (Post Scouring). You are playing a members of the Black Powder and Firearms Guild (AKA Gunslingers Club), or at least have been hired by them. Currently you are the Clubs enforcers.
Many days ago a strange creature arrived at the Gunslingers Club in the town of Goldcrest. He managed to persuade some of the resident tinkerers into giving up their secrets of firearms and put together plans for a unique gun called a 'Hellfire Rifle'. He then acquired the best parts from the club, built the weapon, and left.
The 'Hellfire Rifle' has been declared an illegal weapon by the club, due to the weapon containing evil magic as part of its operation.
The Enforcers have been tracking down this creature for the part few weeks, the journey taking the team to an abandoned volcanic island in The Exuberant Ocean. Near the peak of the mountain, the trail leads to a fiery arcane gate.
Following the trail, the party has figured out that the creature is either a Genasi or Elemental with affinity for fire.

Character Creation

Races
All races are available. As always, check out The Races of the Second Oriana Imperium for what races are the most common.
Classes
For this particular adventure everyone will have 9 levels of Fighter. Everyone is required to choose the Matthew Mercer Gunslinger 1.3 subclass, available on the DMsGuild. This is a 'Pay What You Want' PDF, can be found here: https://www.dmsguild.com/product/170778/Gunslinger-Martial-Archetype-for-Fighters
Starting Level
The starting level is 9.
Starting Equipment
Characters start with the following equipment, awarding them to the character once it has been created at level 9:

• Starting equipment and gold as per the character class and background choices.
• Any 1 Common Magic Item* in Roll20
• Any 1 Uncommon Magic Item* in the Dungeon Master's Guide
• 1,800 gp (can be used to buy weapons, armour, adventuring gear, and spellcasting components)
• 1 Potion of Greater Healing
• A set of Tinkers Tools
• A 'Bad News' or 'Hand Mortar'

*Any magical weapons cannot be a Firearm. However, you may give up your Uncommon Magic Item to make one of your Firearms magical with the sole effect of 'The damage this weapon deals is considered magical for the purposes of nonmagical resistances'.
Buying Firearms
The Firearms that are available to purchase are the ones with a listed cost in the Gunslinger 1.3 PDF. All ammunition is available for purchase from the Gunslinger 1.3 PDF.