When crafting firearms, Simple Firearms start at 2d6 damage with the Ammunition and Reload (5) properties. Advanced Firearms start at 2d6 damage with the Ammunition and Reload (5) properties plus an additional property. You can add weapon properties to the weapon, but doing so reduces the damage by one step (see the "Damage Dice Progression" chart) unless otherwise stated. Some of the properties listed here are also in the standard crafting rules, these are properties that can be added to firearms as well and are listed here for your convience.
These weapon properties can be applied to any firearm.
Brutal
Standard weapon property
Components: Any weapon
This weapon gains the Brutal property.
Brutal. This weapon deals one additional dice of damage on a critical hit.
Concealed
Standard weapon property
Components: Weapon with the Light property
This weapon gains the Concealed property.
Concealed. While stowed, you have advantage on Stealth (Dexterity) rolls made to conceal this weapon.
Dry
Standard firearm property
Components: Any firearm
This weapon gains the Dry property. Increase the damage die of the firearm by one step.
Dry. If this weapon is ever submerged in water or doused with a significant quantity of water, it jams. A jammed firearm can’t be used to make an attack until you spend 4 AP to clear the weapon malfunction.
Extended Magazine
Standard firearm property
Components: Firearm with the Reload property
The Reload capacity of this weapon is doubled. The cost of the weapon increases by 25%.
Heavy
Standard weapon property
Components: Weapon without the Swift, Light, or Versatile property or a shield
This weapon gains the Heavy property, and its damage die increases by one step. Shields have 1 more armor, but also add Bulk and 1 capacity to the shield.
Heavy. You have disadvantage on attack rolls with heavy weapons or shields if your Strength is less than 2. Heavy melee weapons may use strength for both the attack and damage rolls.
Light
Standard weapon property
Components: Weapon that doesn’t have the Two-Handed or Versatile properties
This weapon gains the Light property.
Light. A light weapon is small and easy to handle, making it ideal for use when fighting with two weapons.
Reload
Standard firearm property
Components: Ranged weapon with the Ammunition property
This weapon becomes a Firearm and gains the Reload (5) property and the damage dice doubles if it was not already. If the weapon had the Loading property, it loses the Loading property.
Reload. This weapon can be used to make a number of attacks before it must be reloaded. If you have aptitude with the weapon it takes 1 less Action Point to use the weapon, but takes 2 Action Points to reload that weapon plus 1 for each piece of ammunition. If you do not have aptitude with the weapon, it takes 4 Action Points to reload it per 3 pieces of ammunition. Reloading takes two hands. Reload weapons do not add your attribute to the damage roll.
Two-Handed
Standard weapon property
Components: Any weapon that doesn’t have the Light, or Versatile properties
This weapon gains the Two-Handed property and its damage die increases by one step.
Two-Handed. This weapon requires two hands to use. This property is relevant only when you attack with the weapon, not when you simply hold it. Two-handed weapons cost 2 more Action Points to use.
You can apply one Apprentice property to a weapon.
Automatic
Apprentice firearm property
Components: Any firearm
This weapon gains the Automatic property and its damage decreases by one step. The weapon’s normal range decreases by 20 feet and its long range decreases accordingly.
Automatic. When you make an attack with this weapon on your turn, you can choose to make two attacks instead. These attacks are always made with disadvantage, regardless of circumstance. These attacks use twice the normal amount of ammunition.
Foregrip
Apprentice firearm property
Components: Any firearm
This weapon gains the Foregrip property. If the weapon had the Two-handed property, its short and long range decrease by half and it loses the Two-handed property.
Foregrip. This weapon can be used with one or two hands. If used in two hands, its normal and long ranges double, in addition, the Action Point cost of using the weapon two-handed is only increased by 1 instead of 2.
Scatter
Apprentice firearm property
Components: Firearm with the Two-Handed property that doesn’t have the Sighted property
This weapon gains the Scatter property and its damage die decreases by one step.
The weapon’s normal range is halved and its long range becomes two times the weapon’s normal range.
Scatter. If you make an attack against a target that is within half this weapon’s normal range, the value in parentheses is applied to the weapon. If a number is listed, the weapon uses the value listed instead of its base value.
Sighted
Apprentice firearm property
Components: Firearm that doesn’t have the Scatter property
This weapon gains the Sighted property. Additionally, its normal range increases by 40 feet, and its long range increases accordingly.
Sighted. This weapon has disadvantage on attack rolls made against targets within 20 feet. The Action Point cost of aiming is reduced by 2 Action Points.
Twinshot
Apprentice weapon property
Components: Ranged weapon with the Loading or Reload property
This weapon gains the Twinshot property.
Twinshot. Once on each of your turns when you make an attack with this weapon, You may spend an extra 3 Action Points to fire twice, doubling the dice rolled.
You can apply one Journeyman property to a weapon. You can’t apply a Journeyman property to a weapon that already has one.
Explosive
Journeyman firearm property
Components: Ranged weapon
This weapon gains the Explosive property and its damage die decreases by one step. If this weapon dealt bludgeoning, piercing, or slashing damage, the explosion deals fire damage instead.
Explosive. When this weapon’s projectile hits a target, it explodes in a 5-foot radius. The projectile can target an unoccupied space within its range. The projectile can be fired at an unoccupied space within its range. Each creature other than the target within the blast radius must make a Dodge roll against a target 14 attack.
Heat
Journeyman firearm property
Components: Masterwork Firearm
This weapon gains the Heat property and its damage die increases by one step.
Heat. This weapon gains 1 heat point whenever an attack is made with it, and it loses 1 heat point at the start of each of your turns. If the weapon has 3 heat points, it overheats and loses all heat points. An overheated weapon can’t be used to make an attack again until the start of the next round.
Mounted
Journeyman weapon property
Components: Ranged weapon with the Heavy property
This weapon gains the Mounted property and its damage die increases by one steps. If the weapon had the Two-handed property, it loses the Two-handed property.
Mounted. This weapon is normally used while attached to a tripod, vehicle, or other bracing mount. While it is mounted, it can’t be moved. You can mount or unmount this weapon for 5 Action Points. It can only be used to make an attack while unmounted if held by a Medium or larger creature in two hands with a Strength of at least 3. You move at half your movement while holding this weapon.
Siege
Journeyman weapon property
Components: Ranged weapon with the Loading or Reload property that doesn’t have the Scatter property
This weapon loses the Loading or Reload property and gains the Siege property. Most siege weapon require the Mounted property.
Siege. This weapon takes 10 Action Points to reload per 2 dice of damage. You can reduce the Action Point cost by 6 Action Points for every person helping, over the minimum needed to operate the weapon. The total reload cost cannot be reduced below 5 Action Points. Siege weapons deal double damage to objects and structures. The first number is the minimum number of people needed to use the weapon, and the second number is the maximum number of people that can contribute to the reload. If no number is listed, only one person is needed to operate and only one more person can assist with reloading.
Sniper
Journeyman firearm property
Components: Firearm that doesn’t have the Scatter property
This weapon’s long range becomes eight times the weapon’s normal range.
You can apply only one Master property to a weapon at a time.
Deadly
Master firearm property
Components: Masterwork firearm
When you make an attack with this weapon as part of an action, you can add your Acuity to its damage rolls.
Rifled
Master firearm property
Components: Masterwork firearm
Special grooves have been cut in the bore of the weapon. You double the short range of this weapon but do not increase the long range of the weapon.
You can apply only one Legendary property to a weapon at a time.
Penetrating
Legendary firearm property
Components: Masterwork Firearm or Siege weapon that deals piercing damage
This weapon pierces through its targets. When you make a ranged attack with this weapon and don’t have disadvantage, you can target all creatures in a straight line within this weapon’s normal range. Make one attack roll against all creatures in that line.
Puncturing
Legendary weapon property
Components: Masterwork weapon that deals piercing damage
This weapon ignores the damage resistance from the armor of the creature it hits.
The follow enchantments can to be placed on firearms.
Guiding
Apprentice weapon enchant
Components: Any weapon
This weapon becomes magical and gains a +1 bonus to its attack rolls.
Blessed
Journeyman weapon enchant
Components: Any weapon
This weapon deals an extra 1d4 fire damage on a hit. This extra damage increases to 1d10 fire damage if the target is a Fiendish or Undead.
Cursed
Journeyman weapon enchant
Components: Any weapon
This weapon deals an additional 1d6 necrotic damage on a hit. This additional damage increases to 1d12 necrotic damage if the target is divine.
Guiding
Journeyman weapon enchant
Components: Any weapon
This weapon becomes magical and gains a +2 bonus to its attack rolls.
Elemental
Master weapon enchant
Components: Any weapon
When you apply this property, choose acid, cold, fire, or lightning damage. This weapon deals an extra 1d6 damage of the chosen type on a hit.
Guiding
Master weapon enchant
Components: Any weapon
This weapon becomes magical and gains a +3 bonus to its attack rolls.
Slaying
Master weapon enchant
Components: Any weapon
When you apply this property, choose one creature type other than Humanoids. This weapon deals an extra 1d12 damage to creatures of the chosen type.
Vampiric
Master weapon enchant
Components: Any weapon
This weapon deals an extra 1d4 necrotic damage on a hit. When you hit a creature with this weapon, you regain health equal to the extra necrotic damage dealt by this property.
Seeking
Legendary weapon enchant
Components: Any ranged weapon
When you make an attack roll that doesn’t have disadvantage with this weapon and miss, you instead hit the target and deal 5 damage of the weapon’s type. This damage can’t be increased in any way, and no other effects can be applied to the hit.