The following homebrew adds a modern option to the Monk class in 5E TTRPGs. All of the core Monk class abilities can stay rules as written; see the official 5E System Reference Document (SRD v5) for details. Note: This subclass has not yet gone through playtesting.
Gun-Fu Killer
(AKA Gun Kata, or Way of the Gun)
You’ve mastered the art of death as discipline. Whether forged in black-ops programs, a criminal underworld, or an ascetic order of assassins, you’ve learned to move, shoot, and strike as one fluid motion. To you, violence is not chaos, it’s choreography. You meditate through the rhythm of reloads, the breath between shots, and the stillness after the kill.
Level 3: Gun-Fu Discipline
You blend firearms and martial arts into a seamless form of combat.
Firearm Proficiency. You gain proficiency with all firearms. Firearms you wield count as monk weapons for your features and abilities.
Fluid Reload. If you make an unarmed strike as part of your Attack Action, you can reload a firearm you’re holding as a free action once per turn.
Close-Quarters Reflexes. When a creature enters your reach, you can use your Reaction to make either an unarmed strike or a firearm attack against that creature.
Ki Shot. You can spend 1 Ki Point to steady your aim with perfect precision. Until the end of your turn, firearm attacks you make ignore half and three-quarters cover, and your firearm attacks score a critical hit on a 19 or 20.
Level 6: Centerline Execution
Your movements flow like geometry. Every angle, every target measured and memorized.
Two-Weapon Flow. When you take the Attack Action and attack with an unarmed strike or monk weapon, you can make one firearm attack as a Bonus Action.
Disarm and Drop. When you hit a creature with a melee attack, you can spend 1 Ki Point to attempt to disarm it. The target must succeed on a Strength saving throw (DC 8 + your Proficiency Bonus + your Dexterity modifier) or drop one weapon it’s holding.
Momentum Reload. Each time you reduce a creature to 0 hit points, you can reload one firearm without expending an action.
Level 11: Stillness in Motion
You enter a heightened combat trance where the world slows to your rhythm.
Evasion and Aim. When you use your Deflect Missiles feature, you can spend 1 Ki Point to make one firearm attack as part of the same Reaction.
Flow State. When you take the Attack Action, you can spend 2 Ki Points to gain a second Bonus Action that turn; it can only be used for an unarmed strike, reload, or firearm attack.
Gun Meditation. You center yourself behind your weapon’s sights. While holding a firearm, you can use your Patient Defense feature without expending Ki if you haven’t moved more than 10 feet this turn.
Level 17: Relentless Executioner
You are a living weapon of perfect control.
Death Zone. When you use Flurry of Blows, you can replace any number of unarmed strikes with firearm attacks. These firearm attacks can target any creature within 10 feet of you without disadvantage, even if they are in melee range.
No Missed Motion. When you reduce a creature to 0 hit points, you can immediately move up to half your Speed and make one unarmed strike or firearm attack as part of the same Action, without expending Ki or provoking opportunity attacks.
Zen Reload. At the start of each of your turns, if all firearms you wield are empty, you can reload one of them for free.