An Arcane Marksman is a ranged class that focuses on single target damage.

 

Firearm Proficient

You are proficient in the use of firearms. Moreover you can magically craft ammunition for you firearm. You can produce a number of ammunition equal to your Int modifier + your level in this class in 2 hours.
Additionally, you can choose to use your Intelligence modifier instead of Dexterity when using firearms.

Marksmans Grudge

At level 3 you learn how to mark your target for destruction. Once an enemy has been hit by your firearm attack the Marksman's grudge is placed on them. Every subsequent attack adds an additional +1 bonus to hit and damage until the grudge reaches +3. The next successful attack on the same target [4th attack]  is a critical hit and the grudge is removed. 

Extra Attack

At 5th level, when taking the attack action you attack twice instead of once.

Arcane Ammunition

Once per turn, when taking the attack action, you can imbue your ammunition with the effects of an offensive cantrip from the Artificer list that has a casting time of 1 action. You can use the feature a number of time equal to half your proficiency bonus rounded up. If the cantrip requires a save, it automatically succeeds.  

Critical Execution

At 15th level, when you score a critical hit you can make one additional attack against your opponents. Moreover, at 15th level, your critical range increases to 19-20.