While working on my Urban Arcana stuff, I've been considering the (re)introduction of the ballistic damage type. The DMG appendices referencing firearms list has them dealing piercing damage, which is fairly appropriate for the abstraction of 5th Edition combat. I can't pretend my layman's knowledge of modern firearms would stand up to professional scrutiny, but fortunately 5E combat doesn't concern itself with the stopping power of a specific gun/ammunition combo.
In d20 Modern, the ballistic damage type was its own thing, separate from the usual bludgeoning, piercing, and slashing physical damage types. There's something to be said for attaching a new damage type so that rider effects can be placed on it, without needing to heap on piles of conditional text.
That's a little vague. Basically, what I'm saying is that I could introduce spells, magic items, and technological gadgets that improve or provide resistance against ballistic damage without needing to say "piercing damage sourced by firearms from categories X, Y, and Z" every time I need the conditional effect. It could also help provide granularity for certain kinds of modern protective gear (such as certain styles of ballistic vests that are good for stopping bullets, but not necessarily knives).
Of course, there's some baggage involved with introducing a new damage type outside of the ones already provided. We've already got piles of them. It may warrant a general guideline with monsters and encounter building to say that if a monster is immune or resistant to non-magical bludgeoning, piercing, and slashing damage, it's thereby inferred that they are also resistant to non-magical ballistic damage.
My working documents are going to make the assumption that ballistic damage is a thing. My revised firearms tables will also take this into account, though they're still going to be grouped into fairly broad categories. The last thing I need is an internet argument with strangers over just how much better gun X is than gun Y of the same model, because frankly I don't know guns super well and 5E doesn't need that level of granularity.
That said, even the DMG's firearm damage is gonzo. Chances are extremely high -- with very valid justification -- that a 1st level character is going to be taken from full HP to zero with a single shotgun blast, not to mention a high-powered rifle. Encounter design should be handled judiciously for such campaigns, of course.
Anyway, I'm curious what others think. Am I missing something obvious with introducing a new damage type?