It seems to me that for about as long as people have tried to use guns in DnD, developers and homebrewers alike have been deadly afraid to make them any good. I suppose this makes sense; generally people come to DnD for castles, dragons, swords, magic, that sort. But many people think that guns have a rightful place in a fantasy society. A human wizard can open a gate to the unholy beyond to capture and bind a demon, but an accomplished gnomish alchemist cannot figure out that sulfur, saltpeter, and charcoal explode, and then an accomplished dwarven artificer cannot figure out that if you put it in a tube and but a ball in front of it, it'll shoot out? A lot of fantasy settings are in a sort of equivalent of the High Middle Ages, so a few guns here or there shouldn't be that surprising to see. But this is all another argument, so I digress.
It seems a common sentiment of this playtest is the same thing I noticed above: Guns mechanically have been weak. Some of these reflect real-world problems with old guns: long load times, difficult to obtain, and very, very loud. But some others don't make as much. 20 feet of range? That doesn't quite seem right unless the guns are smoothbore. 1d8 damage? Well I understand that making the die entirely too big wouldn't be fair, but it is a soft lead ball traveling near the speed of sound that fragments and bounces about the soft squishy parts when it hits. Touch AC? Kinda unfair to gun users and everyone else at once in that it helps hit big things that might have high ACs, but makes it hard to Deadly Aim on damage when adding damage to ranged has sort of been a pickle in its own right. So let's take a look at what we can do.
I won't claim to be the most accomplished DM or homebrewman out there, but I have a little experience under my belt. I'm sure a lot of you out there have a lot more. So let's share some ideas, see if something comes out of it.
I'll try my hand at it just because. Once guns turned from handcannon to viable weapon, they became Crossbow Plus. So there probably shouldn't be anything a crossbow does that a gun can't do better save for maybe reload times and range, initially. Remember, unless you've taken a feat or are a Gunslinger, you aren't going to see much use of a gun and they do have drawbacks, so making them useful isn't a bad thing. 1d8 for a one-handed range is about as big as you can go, but I think that a pistol should still out-damage a light crossbow, so let's make that 2d4. Maybe two damage die can be a running theme with guns? 20 foot range. Even a smoothbore pistol could probably fire longer than that, so let's say 40. Rifling has been around since the 1500's, so we can assume that engineers with access to magic can do it, but it's still an expensive process. Let's say that masterwork guns have double range because they are rifled. So a rifled pistol has a range of 80 feet. A musket might have a range of 60 feet and a masterwork musket (a rifle) has a range of 120 feet.
Misfires, another can of worms. Yes, guns jam. And because guns should have higher power and are mechanical, maybe jamming is a viable mechanic. But a jammed gun rarely explodes and a musket doesn't jam any more than a pistol does. So let's just have a flat jam rule of a rolled 1 jams the gun, takes a standard action to clear. If revolvers are implemented, maybe they get a 1 or 2 just because you can immediately fire another round, but even then it's a bit wonky. Misfires are a rule that unfairly persecute one weapon over another.
Touch attacks. While this is outwardly a pretty cool mechanic, it's just a weird rule that has caused overcompensation in gun design elsewhere. If I had a say, I would just cut it out completely. This is sort of fudging reality a little bit, but it's more trouble than it's worth and armor does have an effect on if a bullet hits or not.
Here's my take boiled down:
Pistol: 2d4 Medium (1d6 small) - x4 - 40 ft (80 ft mstwrk) P and B, jams on a 1 (maybe)
Musket (Rifle): 2d6 Medium (1d10 small) - x4 - 60 ft (120 ft mstwrk) P and B, jams on a 1 (maybe)
I would also perhaps include revolvers (maybe even revolver rifles, those did exist) and blunderbusses/scatterguns that were represented in the campaign setting. The extra jam on revolvers make them viable on the power scale I would think, especially because they would take so long to reload. If that wasn't enough, going down a die on damage might be feasible. But hey, come share your own ideas, I'm sure I'm completely wrong somewhere and would like to know.