- A property might need to be broad enough to cover similar properties of entire groups of firearms.
- It also needs to be simple to remember.
- It also needs to not break the hell out of the game.
That said, there's a lot of reasons firearms supplanted the archer, the knight, and the pike. Their incredible killing potential aside, it's really all about a relatively light amount of physical prowess required to kill a whole lot of dudes way over there. So it's okay that guns are dangerous in 5E Urban Arcana.
Anyway, here's a rough draft of what I've come up with.
An automatic firearm chambers a new round without needing to remove one's finger from the trigger, and can be used to perform two additional attack modes. When you make an attack using an automatic firearm, you can opt to perform a three-round burst. This attack grants you advantage on the attack roll, but consumes three rounds from your ammunition. You cannot perform a three-round burst if you do not have at least three rounds remaining in the firearm's magazine.
You may also attempt to perform a spray with a full magazine. The entire magazine is consumed when you make this attack. In lieu of making attack rolls, you may force a number of creatures in a cone with a range equal to the weapon's medium range to make a Dexterity saving throw or suffer the firearm's damage. The number of creatures you can affect is equal to the number of bullets in the magazine. The DC for this saving throw is 8 + your Dexterity modifier + your proficiency bonus.
Most weapons are not automatic by default, and this property is typically bestowed upon a weapon by using a weapon modification kit. Automatic firearms are illegal in nearly all civilian environments, and owning one without sufficient clearance is grounds for arrest, confiscation, and/or fines by Earthly law enforcement entities.
Weapons with the loud property cannot be used for their traditional attack without causing a great deal of noise. Whenever you make an attack with a loud weapon that is not benefiting from a silencer or other means of suppressing noise, the sound of the attack can be heard up to 1000 feet away in open ground, or 300 feet away inside a typical building.
Unlike the more punitive loading quality, you may make as many attacks as the weapon has in its magazine before you must use an action to reload the weapon.
Of course, a trained firearm expert is going to be able to reload a weapon rather quickly. Thus I also include a feat:
Effect: Your Dexterity score increases by 1. You may replace the magazine on any weapon with the reload quality as a bonus action rather than an action.
Now, there's something to be said for some weapons not being able to load quite as fast as that. Loading up some kinds of shotguns might take you longer than the couple seconds implied in a bonus action (I know I'm inviting myself to be eviscerated by someone saying they can do it faster, but I am willing to eat crow on that), but like I say in the design goal bullet points, I'm trying to be broad and still reasonably accurate here.
Same with the Loud quality. I'm completely spitballing on how far away you can hear a gunshot. If you're in an open desert in the dead of a still night, you can hear gunfire from quite far away. I leave those ranges open for correction, but incredibly loud firefights do provide some impetus to get close and dirty with your magic sword against the bugbear mafia when there's a god-baby with an apocalyptic scream sleeping in the crib upstairs.
That may or may not have happened in the old d20 Modern game I ran. Just saying.
Other things I may need to consider are ammunition type, since shot is different from slug which is different from sabot... you see where this rabbit hole leads. I need to be careful to remain true to 5E design goals and not turn firearms into an abyss of corner cases and specialty rules.
I've already got a bunch of weapon mod ideas for them and that's going to be bad enough.
As a bonus, I've converted a couple of spells so far from the old rules and introduced one new spell that I used in my old campaign (silver was important for overcoming damage reduction since magic weapons were still incredibly rare and being smuggled in from the magic world).
Casting Time: 1 action
Range: 120 feet
Components: V, S, M (the cover of a camera lens)
Duration: Concentration, up to 10 minutes
You suppress the functions of electronic or mechanical alarm systems and sensors in a 15 foot radius centered on a point within range. The affected systems include, but are not necessarily limited to, motion detectors, pressure sensors, laser grids, and video cameras.
Cameras continue to broadcast the last thing they saw before the spell was cast in the area, but this spell does not create a visual "dead zone" if the camera itself lies outside of the range of the spell.
Casting Time: 1 action
Components: V, S
You touch a device that contains electronic data, such as a computer or flash drive. All data upon that object is immediately and irrevocably erased.
Steel to Silver
Casting Time: 1 action
Components: S, M (a silver object about the size of a fingernail)
Duration: Concentration, up to 1 hour
You touch a single weapon, 20 arrows, 20 crossbow bolts, or a single magazine of bullets. For the duration of the spell, the weapon takes on a silvery sheen and strikes as though it were crafted of pure silver without sacrificing any of its durability or penetrating power.
At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, you may affect one additional weapon as outlined above for each spell level above 1st.