Eventually the musket is overtaken by technology and cylinder feeds with cartridge ammunition become commonplace. If you have a region of your setting where this might have happened (in the Realms of Chirak the kingdom of Abraheil has reached this point) then the following rules will let you add more advanced firearms.
Advanced Firearm Properties:
Cartridge/cylinder-load firearms are much quicker to load and rifling is now the default process when constructing these weapons. Gatling guns become possible, wheel-mounted monstrosities which crank out a hail of bullets. Advanced firearms can fire a number of shots before reloading. Once the number of loaded rounds has been expended then the weapon must be reloaded. The number of actions it takes to reload is the number following “reload.” Most are Reload 1 or 2.
Cartridge/cylinder-load firearms can misfire, and the same rules apply. However wet cartriges are uncommon unless the weapon has been immersed in water and there’s considerably more reliability over flintlock weapons. When rolling a 1 on an attack make a DC 10 saving throw for the weapon. If you fail then the weapon is jammed and required 1D3 actions to unjam. If you roll another 1 the weapon’s misfired and if the shooter fails a DC 12 Dexterity check he will take 1D8 damage from the round going off in the chamber. The weapon is then useless until repaired.
The cylinder loaded weapons have a number of shots that can be fired before reloading, typically 6 or 8. You can purchase a contraption for quick loading of a cylinder loader for 10 gp which reduces the weapon’s reload rate by 1.
Advanced firearms with iron sights can grant a significant advantage if you take a moment to aim. With this property, a shooter can spend a move or regular action to aim down the iron sights to gain advantage on the target.
Advanced Firearm Stats:
Revolver (250 GP)
Properties: Ammunition, 6 rounds, Advanced Misfire, Reload 2, Armor Piercing, deadly crit, Iron Sights, Range (50/100), light one-handed
Revolvers are reliable weapons, with a spin cylinder which loads the brass cartridge rounds. You can purchase a revolver with an 8 round cylinder for 500 GP. Weight 6 lbs.
Breach-Loading Rifle (800 GP)
Properties: Ammunition, Misfire, Reload 1, Armor Piercing, deadly crit, Iron Sights, Range (200/400), two-handed
Breach-loading rifles require that the shooter insert a new bullet after the old cartridge is released. It requires a load action between shots, but a professional with the firearms style can load a shot as part of a move action. Weight 10 lbs
Revolving Rifle (1,200 GP)
Properties: 8 rounds, Ammunition, Misfire, Reload 2, Armor Piercing, deadly crit, Iron Sights, Range (200/400), two-handed
Revolving rifles use the cylinder loader to hold a number of rounds for continuous fire. Add 80 GP to the cost for each additional round it can hold up to 15. Weight 10 lbs
Shotgun (500 GP)
Damage: 2D8 short range, 2D4 long range
Properties: ammunition, 2 rounds, Misfire, Reload 1, deadly crit, Range (30/60), two handed
The shotgun evolved from the blunderbuss. This weapon fires lead shot for great effectiveness at close range, with damage dropping at greater range. Most shotguns have 2 barrels, but you can get a single barrel version for 250 GP, or a four-barrel version for 1,000 GP. Weight 5 lbs
Gatling Gun (25,000 GP)
Damage: 1D8 per bullet
Properties: ammunition, 100 rounds, Misfire, Reload 4, Armor Piercing, deadly crit, Range (100/200), heavy, two-handed (mounted only)
The first Gatling guns were positioned on tripods or wheels with a frame, and aimed in the general direction of the enemy, with a crank to funnel bullets into the weapon while turning the cylinder that both loads and fires through the active barrel.
To fire a Gatling gun, roll a single attack against all targets in the effective range (choose to fire in a 10/10 cylinder or 5X20 cone) and each target that you hit takes 1 bullet for every 3 points rolled over their effective armor class. You roll an additional die for disadvantage against any targets at long range.
Note that this is a heavy weapon and not easily maneuvered; features like the sharpshooter feat do not apply to this weapon. Weight 200 lbs. (proficiency in land vehicles will allow one to maneuver the gun as a wagon with a horse or other beast of burden.)
Cartridge Bullets (50 GP)
100 brass cartridge rounds which will work with both pistols and rifles. Weight 5 lbs.
Gatling Rounds (100 GP)
100 heavy rounds linked and loaded into a feeder box. Weight 25 lbs.
Shotgun Rounds (25 GP)
A box of 50 lead-shot loaded paper cartridges for shotguns. Weight 5lbs.