My game has renaissance firearms (Chapter 9: DMG, Firearms) and the rogue is proficient with a Pistol (1d10 piercing, 3lb, Ammunition (range 30/90), loading.
He has a custom feat that works similar to Crossbow Expert:
Through a combination of smithing, tinkering and ingenuity, you've discovered how to craft firearms.
You gain proficiency with firearms. You can now ignore the loading property of one-handed firearms. Being within 5 feet of a hostile creature doesn’t impose disadvantage on your ranged attack rolls with one-handed firearms.
He's also attempting to use Two-Weapon Fighting to wield a shortsword at the same time. (Chapter 9: PHB, Combat, Two-Weapon Fighting)
When you take the Attack action and attack with a light melee weapon that you're holding in one hand, you can use a bonus action to attack with a different light melee weapon that you're holding in the other hand. You don't add your ability modifier to the damage of the bonus attack, unless that modifier is negative.
If either weapon has the thrown property, you can throw the weapon, instead of making a melee attack with it.
From what I understand, he cannot dual wield a Renaissance Pistol and a Shortsword as 1) They aren't both light, and 2) They aren't both melee
My question is what should I do to follow the rules more closely, as well as help the rogue out? This style has become part of their character.
I found this for the Crossbow Expert Feat:
When you use the Attack action and attack with a one-handed weapon, you can use a bonus action to attack with a hand crossbow you are holding.
How do I modify this feat to get the two weapon pistol and sword style to fit better?
The character concept is a swashbuckling sword and pistol rogue. I feel that if I just added the above text, the feat would be overpowered.
Alternatives that I have looked into and that I don't really like (hence this question):
I am considering nerfing his pistol from 1d10 piercing to 1d8 piercing and giving it the light property so that he can dual wield it, even though it's not melee.
Ignore the fact that it's not melee;
Stick to RAW and tell him too bad the game doesn't work that way?