Apostate

You have a +3 bonus to attack and damage rolls made with this magic weapon.

The Apostate is a double-barrel rifle of immense size. It follows the rules of a cannon. One action to aim, one action to fire, and one action to load a barrel, however, after firing the first barrel you can use a bonus action to aim the second barrel.

This weapon is explicitly made to hunt giant creatures. With a bore of 4, this rifle has a bore diameter of slightly over an inch. Made with Mithril, It does not misfire.

4d12 is the damage roll for the standard ammunition.
With the powder charge and bullet weight being about 4 to 5 times the size of the usual musket charge, the damage dice change is perfectly adequate.

If special ammunition is made for the Apostate, it costs three times as much to make than it would if it were regular musket ammunition.

Because of the sheer size of the ammunition fired from this weapon at least a +3 in strength is required to wield the gun, and a flat +5 damage is to any successful hit.

so it works out to 4d12 + DEX + 3 + 5 on your damage rolls for standard ammo.

Firearms are a new and volatile technology, and as such bring their own unique set of weapon properties. Some properties are followed by a number, and this number signifies an element of that property (outlined below). These properties replace the optional ones presented in the Dungeon Master’s Guide. Firearms are ranged weapons.

Reload. The weapon can be fired a number of times equal to its Reload score before you must spend 1 attack or 1 action to reload. You must have one free hand to reload a firearm.

Misfire. Whenever you make an attack roll with a firearm, and the dice roll is equal to or lower than the weapon’s Misfire score, the weapon misfires. The attack misses, and the weapon cannot be used again until you spend an action to try and repair it. To repair your firearm, you must make a successful Tinker’s Tools check (DC equal to 8 + misfire score). If your check fails, the weapon is broken and must be mended out of combat at a quarter of the cost of the firearm. Creatures who use a firearm without being proficient increase the weapon’s misfire score by 1.

Explosive. Upon a hit, everything within 5 ft of the target must make a Dexterity saving throw (DC equal to 8 + your proficiency bonus + your Dexterity modifier) or suffer 1d8 fire damage. If the weapon misses, the ammunition fails to detonate, or bounces away harmlessly before doing so.

Ammunition

All firearms require ammunition to make an attack, and due to their rare nature, ammunition may be near impossible to find or purchase. However, if materials are gathered, you can craft ammunition yourself using your Tinker’s Tools at half the cost. Each firearm uses its own unique ammunition and is generally sold or crafted in batches listed below next to the price.

THIS IS UNOFFICIAL MATERIAL

These game mechanics are usable in your campaign if your DM allows them but not refined by final game design and editing. They aren’t officially part of the Dungeons & Dragons game and aren’t permitted in D&D Adventurers League events unless otherwise stated.

Notes: Bonus: Magic, Weapon Property: Versatile, Damage: Piercing, Combat, Two-Handed, Reload, Misfire