The biggest flex there is in D&D, a red money gun that kills.

When firing the money gun, instead of bullets it shoots American 100 dollar bills that deal only 1d6 psychic damage instead of 2d6 piercing damage but you can shoot it up to 3 times with one attack. Whenever you score a critical hit with the money gun or reduce a creature to zero hit points with it, they turn to gold and you get a misprint dollar. You can expend a misprint dollar to use the following options bellow and you can have up to 500 dollars at once.

Make it Rain (Costs 20 Misprints, Bonus Action). You fire the money gun into the air causing a downpour of a lot of cash, probably more than you've ever seen. Each creature within 15 ft. of you must make a DC 20 wisdom saving throw or be charmed by you for one hour or until you deal damage to them. If you use this property on an already charmed creature it takes 5d6 psychic damage and is charmed for a day or until it takes damage again.

Buy Me Something Buddy (Costs 10 Misprints, Action). You command a creature within 30 ft. of you to get you something, they must make a DC 20 saving throw (Charmed creatures have disadvantage) or drop whatever they're holding and offer it to you.

Counterfeiting (Costs 50 Misprints, Action). You conjure one knockoff currency of your choice with a maximum of 50,000 GP in worth. It can be passed off and used as real money but by spending it you summon 1d4+2 I.R.S. agents (Stat block veterans) in an area within a mile of you that will track you down and attempt to kill you.

I'm Too Rich to Die! (Costs a minimum of 75 Misprints, Reaction). In response to dropping to 0 hit points, your money saves you causing you to instead drop to one hit point and all your left over money is converted to temporary hit points.

Curse. You're party no longer likes you do to your bragging and money.

It’s up to you to decide whether a character has proficiency with a firearm. Characters in most D&D worlds wouldn’t have such proficiency. During their downtime, characters can use the training rules in the Player’s Handbook to acquire proficiency, assuming that they have enough ammunition to keep the weapons working while mastering their use.

Notes: Requires a net worth of at least 300 GP, Range, Ammunition (Firearms), Reload