Is there an anticipated max character level for this campaign? For high-level fighters and characters with two-weapon fighting, a PC can go through six pistols in only two rounds.
I think that while the mechanic of a Dexterity saving throw works reasonably well as a means to say that bullets fire too fast and hit too hard for armor to be helpful, but it runs into a lot of places where it does not integrate well into the rules. That is going to lead to you needing to highlight for the players which options will not work or write new rules around firearms to patch all the missing bits.
Assuming that shooting a firearm is not an Attack or a Magic action, that leaves you with the Utilize action. I could see all of the following needing attention to integrate this homebrew with the 2024 core books.
Only one firearm per round. Most PCs can't use the Utilize action a second time before the start of their next turn.
Favors certain classes. Barbarians, bards, monks, rogues, and rangers get extra benefit as they either apply their Proficiency Bonus or have Advantage for their Dexterity saving throws.
Integration with class features. Just about every class has either core features or features within at least one subclass that depend on armor, weapons, and attacks (Extra Attack, Unarmored Defense, Divine Strike, warlock pact weapons, Sneak Attack, battle master maneuvers, Hunter Ranger, monk deflect missile, etc.) to integrate with firearms.
Integration with two-weapon fighting. Two-weapon fighting only works with attacks.
Bonus to niche options. Thief Rogues and artificer homunculus can shoot a firearm on their turn as a Bonus Action.
No weapon masteries. Those are tied to attacks.
New item classification needed. Firearms are not weapons, you use a weapon to make an attack.
Equipping and unequipping firearms. Since firearms are not weapons that are used in attacks, a creature has to use its one interaction with the environment to equip or unequip a firearm. This makes firing more than one firearm on a turn.
Integration with feats. Most feats relevant to a user of ranged weapons expect weapon attacks (or crossbows).
Potentially unhappy players. When I was brainstorming new firearm options for my Zeitgeist campaign, my players were very unhappy with the idea of discounting armor and shields, either through Touch AC or Dexterity saving throws.
Spell interactions. Shield, true strike, barkskin, etc., don't work, but stoneskin does.
Some of this can be helped by making firearms weapons with a "special attack". Features and benefits that would normally occur on a hit now occur on a failed saving throw. That could tie in weapon masteries, the Sharpshooter feat, Extra Attack, Sneak Attack, equipping and unequipping weapons, two-weapon fighting, etc.
That causes other considerations.
Potential free damage. A pistolier fighter could fire two pistols with Two-Weapon fighting and their interaction with the environment (three at 5th level with Extra Attack). If firearms do more than 1dX damage (say 2dX), that is 2dX or 3dX free damage on the first several turns of each encounter just for the pistoliers.
Lack of long range. You had mentioned not wanting to have a long range, but ranged weapons have a normal and long range. In this case giving Advantage on saving throws at long range would be similar but essentially make bards, monks, rangers, and rogues very hard to hit. This would help more options like Sharpshooter to fully integrate with firearms.
Mostly tops out at three attacks per round. Attacks let you equip or unequip a weapon as part of the attack, but most PCs only have two hands and one free interaction with the environment. Fighters don't really have the hands or interactions to juggle more than three pistols on a turn, and pistols cease being a great option once the pistolier reaches level 11 for the second extra attack.
Many ways of dealing with attacks still don't work. Aside from the loss of armor, shield, barkskin, parry, and other means of dealing with attacks on a temporary basis don't work.
Still benefits certain niche classes. Thief Rogues and artificer homunculus can probably reload as a Bonus Action and fire every turn.
If I were to make a firearms homebrew that used Dex saving throws I would probably go in the direction of the following to address most of the issues.
Keep the 1-minute reload times. According to Google, a well-trained soldier could reload a musket in 30-60 seconds if they did not have access to paper cartridges. Using the Utilize action to reload and firing every other turn is closer to the reload time for paper cartridges (15-20 seconds). Without paper cartridges, reloading does not make sense to do in combat even under the haste spell. There also is not a significant enough reason to penalize reloading a firearm for creatures not proficient with firearms.
In other words, creatures cannot reload firearms fast enough to make a second shot relevant in combat without advances in technology. Paper cartridges allow reloading as a Utilize action (with the potential "abuses" through haste, Fast Hands, etc.). Metal cartridges allow reloading as an interaction with the environment or a Bonus Action and allow for firearms with the Reload (X shots) property.
I think that while the mechanic of a Dexterity saving throw works reasonably well as a means to say that bullets fire too fast and hit too hard for armor to be helpful, but it runs into a lot of places where it does not integrate well into the rules. That is going to lead to you needing to highlight for the players which options will not work or write new rules around firearms to patch all the missing bits.
Assuming that shooting a firearm is not an Attack or a Magic action, that leaves you with the Utilize action. I could see all of the following needing attention to integrate this homebrew with the 2024 core books.
Only one firearm per round. Most PCs can't use the Utilize action a second time before the start of their next turn.
Favors certain classes. Barbarians, bards, monks, rogues, and rangers get extra benefit as they either apply their Proficiency Bonus or have Advantage for their Dexterity saving throws.
Integration with class features. Just about every class has either core features or features within at least one subclass that depend on armor, weapons, and attacks (Extra Attack, Unarmored Defense, Divine Strike, warlock pact weapons, Sneak Attack, battle master maneuvers, Hunter Ranger, monk deflect missile, etc.) to integrate with firearms.
Integration with two-weapon fighting. Two-weapon fighting only works with attacks.
Bonus to niche options. Thief Rogues and artificer homunculus can shoot a firearm on their turn as a Bonus Action.
No weapon masteries. Those are tied to attacks.
New item classification needed. Firearms are not weapons, you use a weapon to make an attack.
Equipping and unequipping firearms. Since firearms are not weapons that are used in attacks, a creature has to use its one interaction with the environment to equip or unequip a firearm. This makes firing more than one firearm on a turn.
Integration with feats. Most feats relevant to a user of ranged weapons expect weapon attacks (or crossbows).
Potentially unhappy players. When I was brainstorming new firearm options for my Zeitgeist campaign, my players were very unhappy with the idea of discounting armor and shields, either through Touch AC or Dexterity saving throws.
Spell interactions. Shield, true strike, barkskin, etc., don't work, but stoneskin does.
Some of this can be helped by making firearms weapons with a "special attack". Features and benefits that would normally occur on a hit now occur on a failed saving throw. That could tie in weapon masteries, the Sharpshooter feat, Extra Attack, Sneak Attack, equipping and unequipping weapons, two-weapon fighting, etc.
That causes other considerations.
Potential free damage. A pistolier fighter could fire two pistols with Two-Weapon fighting and their interaction with the environment (three at 5th level with Extra Attack). If firearms do more than 1dX damage (say 2dX), that is 2dX or 3dX free damage on the first several turns of each encounter just for the pistoliers.
Lack of long range. You had mentioned not wanting to have a long range, but ranged weapons have a normal and long range. In this case giving Advantage on saving throws at long range would be similar but essentially make bards, monks, rangers, and rogues very hard to hit. This would help more options like Sharpshooter to fully integrate with firearms.
Mostly tops out at three attacks per round. Attacks let you equip or unequip a weapon as part of the attack, but most PCs only have two hands and one free interaction with the environment. Fighters don't really have the hands or interactions to juggle more than three pistols on a turn, and pistols cease being a great option once the pistolier reaches level 11 for the second extra attack.
Many ways of dealing with attacks still don't work. Aside from the loss of armor, shield, barkskin, parry, and other means of dealing with attacks on a temporary basis don't work.
Still benefits certain niche classes. Thief Rogues and artificer homunculus can probably reload as a Bonus Action and fire every turn.
If I were to make a firearms homebrew that used Dex saving throws I would probably go in the direction of the following to address most of the issues.
- Make firearms martial weapons with a Muzzle Loading property. A Muzzle Loading weapon requires a minute to reload and uses a special attack that forces a Dex saving throw instead of an attack roll.
- For rules interactions, the attack is considered a hit if the target fails its Dexterity saving throw. Otherwise, the attack is a miss.
- Any magical bonus that an armor, shield, or other item gives to AC (+1, +2, +3) is still added to the saving throw (because magic), unless it already adds to the Dex saving throw (looking at you ring of protection and cloak of protection).
- If after all sources of Advantage and Disadvantage were taken into account, if you would normally have Advantage on the ranged attack roll, the target has Disadvantage on the Dex saving throw (and vice versa). So tabulate all the things that would normally give Advantage/Disadvantage on a ranged weapon attack (long range, Enemy within 5 feet, Steady Aim, Help, etc.), and then flip the result for the Dex saving throw.
- Start with the pistol and musket in the PHB for the range and damage stats. Any firearms have a long range that is three times their normal range.
- Add a blunderbuss that is functionally equivalent to the musket with the following changes: range of pistol and the Graze Weapon Mastery.
- If a creature can use an ability modifier other than Dexterity to make a ranged weapon attack (Hexblade, true strike, etc.) use that ability modifier to calculate the save DC.
- Provide feats (such as the ones in my first post), that allow a creature to have proficiency in firearms, add their ability modifier/Proficiency Bonus to damage, etc.
Keep the 1-minute reload times. According to Google, a well-trained soldier could reload a musket in 30-60 seconds if they did not have access to paper cartridges. Using the Utilize action to reload and firing every other turn is closer to the reload time for paper cartridges (15-20 seconds). Without paper cartridges, reloading does not make sense to do in combat even under the haste spell. There also is not a significant enough reason to penalize reloading a firearm for creatures not proficient with firearms.
In other words, creatures cannot reload firearms fast enough to make a second shot relevant in combat without advances in technology. Paper cartridges allow reloading as a Utilize action (with the potential "abuses" through haste, Fast Hands, etc.). Metal cartridges allow reloading as an interaction with the environment or a Bonus Action and allow for firearms with the Reload (X shots) property.
