Gunsmith

Magic can be traced back to ancient history. Guns? A little more recent. A Gunsmith brings both of these worlds together to wreak havoc on the battlefield. Make all types of creative firearms and ammunition that have different effects, quickly switch between them in combat, and eventually make your masterpiece... or 5 of them.

The Gunsmith subclass for the Artificer focuses on making and buffing a wide variety of firearm inventions. After a while, they'll go from switching between ordinary firearms to different elements and types of ammunition. One second they could be using a fire pistol with explosive rounds, the next they're pulling out a frost rifle with poison-filled bullets. The sky's the limit with this subclass. So what if they have resistance and immunities? Those don't exist if you have enough firearms. And just wait until the Gunsmith pulls out their mastergun.

Firearm Savant

Firearms are your cup of tea and the focus of your research. You gain proficiency in using firearms if you don't have it already.

Tool Proficiency

When you adopt this specialization at 3rd level, you gain proficiency with tinker's tools. If you already have this proficiency, you gain proficiency with one other type of artisan's tools of your choice.

Requip

You spend an awful amount of time around firearms and your creations. All your firearms are memorized, what they do and where you have them on you. You can use a bonus action to switch firearms during combat. Regardless of what was done before the action, the current firearm in your hand is switched to another of your choosing. If you're dual-wielding, switching to another one-handed firearm will only switch one of the firearms of your choosing. Switching to a two-handed firearm will switch both firearms you are currently wielding.

Damage Smith

You have found a way to create weapons that can infuse your creations' damage with magic, giving every shot extra damage of a chosen type. When creating a new gun, you can choose one type of magical damage (acid, cold, fire, force, lightning, necrotic, poison, psychic, radiant, and thunder) for the gun to infuse with. Any time you roll damage, add an extra 1d4 for one-handed and 2d4 for two-handed firearms. Every two Artificer levels, after level 5, change the die to the next-sided die(i.e. 1d4 to 1d6), up to 1d12 at 13th level.

Ammunitions Expert

Due to making a variety of firearms for so long, you have picked up mastery in the second most important part of the firearm, the ammunition. You can create different types of ammunition out of different types of materials with tinker's tools:

Explosive - Some questionable liquids and gunpowder, with a little magic, make these bullets highly volatile on impact. Extra 1d6 fire damage. Anything explosive that causes an explosion by being shot by this bullet will do the damage of the explosion created by the explosive and not the bullet.
Piercing - The usually round bullet has a long spike coming out of one side. This spike is tipped with a stronger metal and is made to pierce through clothing, skin, tissues, and muscle alike. Add 1d6 piercing damage.
Substance - Often times there's a limited substance on hand with limited applications. Substance bullets made with the substance, liquid, powder, etc., will disperse the substance onto the target. The crafting needs 2 mL of any material substance per bullet (~30 mL to the oz, 4 oz to the vial, 1 vial of poison = 60 poison substance bullets). Substance interactions with the bullet material should be taken into account (i.e. if somehow got lava into a bullet, and it's made of wood the lava will melt through it, and it will be unusable).

Mastergun

After all of your hard work and research, you have made your masterpiece. This firearm has two types of extra magical damage from Damage Smith and can shoot two types of ammo out of each of its two barrels at once. The firearm can be any type of your choosing (pistol, rifle, etc.) and can be created as many times as you like as long as you have the materials. Double the damage of the type of firearm due to the double bullets, which is only applied to the shots and not the extra magical damages. This firearm is complicated and difficult to reload, it takes you 1 hour of short rest to replenish the ammunition. The firearm can hold ammunition equal to 1d6 + your Intelligence modifier.

The specifics of the creation of the mastergun are up to you and your Dungeon Master. But, the creation time must be a minimum of 24 hours of active building and the majority of the materials should be metal or more valuable material. This is the best design you could ever come up with, there can only be one type of blueprint for masterguns, but it can be replicated from said blueprint.

Mastersmith

You have taken the time to become an expert in the creation of your mastergun. You can choose between making a second blueprint for a new mastergun or an extra set of arms that can magically operate masterguns. The second mastergun must be a different type with a different combination of elements and follows all the same rules as your first mastergun but with one more barrel and one more ammo compartment. The arms look doll or robotic-like but are controlled by magic and can only operate masterguns.

Mastergun 2.0

You created a second mastergun using your expert knowledge in your creation. This mastergun is different in type from the original. This firearm has three types of extra magical damages from Damage Smith and can shoot three types of ammo out of each of its three barrels at once. The mastergun can be created as many times as you like as long as you have the materials. Triple the damage of the type of firearm due to the triple bullets, which is only applied to the shots and not the extra magical damages. This firearm is complicated and difficult to reload, it takes you 2 hours of short rest to replenish the ammunition. The firearm can hold ammunition equal to 1d12 + your Intelligence modifier.

The specifics of the creation of the mastergun are up to you and your Dungeon Master. But, the creation time must be a minimum of 24 hours of active building and the majority of the materials should be metal or more valuable material. This is the best design you could ever come up with, there can only be one type of blueprint for Mastergun 2.0s, but it can be replicated from said blueprint.

More Masterguns

The arms look doll or robotic-like but are controlled by magic and can only operate masterguns. The arms cannot be removed by anybody but you, but all physical and magical reactions apply. The AC and Hit Points of the arms are calculated as normal objects are and the hardness is of the material the majority of the exterior is made of. Some examples are:

Dispel Magic will cause the arms to stop working. Instead of DC, the spellcaster must roll against your intelligence saving throw as they are attempting to dispel your infused magic. This roll happens regardless of the level Dispel Magic is cast at as the arms have no spell level.
If broken, they can be fixed with tinker's tools.
If destroyed, they can be recreated and fixed within a short rest.
If whatever damage they have taken has caused them to no longer exist, they can be recreated in an amount of time decided by the Dungeon Master, with a minimum of 24 hours of active building.