Firearm Rules

Guns! How to use em, abuse em, and die by them. Guns are rare, and those guns preserved approach roughly technology levels of the 1870’s. Attempting to design simple mechanics that can be applied to OSR with a few rule-specific changes.

3 types for our purposes: revolvers, rifles, and shotguns. All use exploding dice, don’t add ASI’s to damage, and you can use Dex or Str for attack rolls.

Revolvers:

1d4 (exploding) piercing damage (max 1 dice explosion)

Ammo count: 6 before reload

Range: 50/100

Rifles:

2d4 (exploding) piercing damage (max 2 dice explosions)

Ammo count: 8 before reload

Range 130/260

Shotguns: 3d4 (exploding) piercing damage (max 3 dice explosions)

Ammo count: 2 before reload

Range: 30/60

Special gun rules:

When a gun or ammo is exposed to water, it’s soaked. A soaked gun or a gun using soaked ammo will only be able to fire one shot, after which the gun needs to be stripped and cleaned over a long rest. Soaked ammo can be dried off with a cloth or a spell.

When a loaded gun or ammo is exposed to fire, roll a dice. On an even number, the ammo cooks off dealing 1 piercing damage to the user for every round inside the gun or on the person. The gun itself is ruined unless repaired by a gunsmith. 

When a gun or ammo is exposed to acid, roll a dice. On an even number, the gun/ammo is corroded and won’t work unless repaired by a gunsmith. On an odd number, the gun has a 50% chance of jamming every time it’s fired/corroded ammo has a 50% chance of jamming every time it's used until cleaned over the course of a short rest.

Revolver rules: People proficient with revolvers can attack with them a number of times per round equal to half their proficiency bonus.  Revolvers don't have disadvantage on ranged attacks within 5 feet of a hostile, and they are light weapons. 

Rifle rules: Rifles have disadvantage on all attacks against a hostile creature within 5 feet. Additionally, a bayonet can be attached to the front of a rifle. A creature proficient with rifles may make a melee attack with an affixed bayonet as a bonus action, dealing 1d4 piercing damage.

Shotgun rules: Beyond 30 feet, a shotgun loses one damage dice every 10 feet. 2d4 at 40-50 feet and 1d4 at 50-60 feet. Shotguns don't have disadvantage on attack rolls within 5 feet of a hostile creature. 

Crit rules: When a nat 20 is rolled on an attack roll, add 1d4 to that attack’s damage. This d4 can only explode once.

Crit fail rules: When a nat 1 is rolled on an attack roll, the gun jams. Clearing a jammed gun is an action.

Backfiring: When a gun or its ammo is soaked or corroded, the gun backfires on a crit fail, dealing 2 piercing damage to the user for each round left in the gun and destroying the gun. 

New weapon proficiency: guns. Separate from all other weapon proficiencies.

New tool proficiency: gunsmith’s tools

New ammo types: pistol ammo, rifle ammo, and shotgun shells

Obviously, these will be retooled as time goes on. What I wanted to emphasize:

Revolvers are designed for medium range engagements. Their strength is multiple shots.
Rifles are long-range weapons. Arguably longer effective range but I'm not bumping their strength up just yet. Playtest first.
Shotguns are close quarter murder machines.

I'm going to develop specialized stats for guns to reflect arms races (1893 semi-auto pistol!) but guns will largely stay magic free. There will be different ammo types (including magical types) and magic guns.