Hiya! You have some good ideas in that, I think making nonproficient users take longer to load the weapon is a great idea. My idea for dnd firearms is a little more advanced, involving revolver style weapons. This is what I came up with a few nights ago when considering this topic for a potential campaign, plus the longer reload idea:
using firearms without proficiency incurs a move action to load the rounds, firearms are superior weapons.
handgun
50gp
+3 hit 1d6
10/20
load minor
1 load = 4 shots
rifle
75gp
+3 hit 1d10
20/40
load minor
1 load = 4 shots
shotgun
75gp
5/10
+2 hit 1d6
load minor
1 load = 1 shot
close blast 3 vs reflex, can only be used for basic attacks
singleshot handgun: 30gp
singleshot rifle: 40gp
The idea was to balance them against bows and crossbows, thereby leaving them as options for people's playstyle, so for the revolver rifle and handgun, which are what I was mainly concentrating on, the minor load for the bullets is only a pidling of difference. I figured it'd be equivalent to loading a crossbow since you're just dropping 4 bullets in the chamber vs pulling back the string as is done on a crossbow. At the same time the damage and ranges are the same for a hand crossbow and a longbow, which are what these were modeled on. The +3 prof bonus is due to them being superior weapons, since it seems *all* superior weapons have this prof bonus. I assume that's because they're superior and require a higher level of expertise to use effectively.
THe singleshot weapons are the same as the revolvers except 1 load = 1 shot, dunno how that'd be used for a ragner's multi-firing capabilities, and is there more for the pirate/buccaneer guys with several pistols on their belts.
To tweak these for black powder use instead of guncotton or equivalent, up the load time to a move action, and add a 1d20 roll when loading the weapon for nonproficient users, on a roll of 1, one of the bullets will automatically fumble somehow. Also keep in mind that yes, revolver style blackpowder existed during the civil war, and revolver style rifles made by colt were shortly commissioned by the US government.
The shotgun is a special case weapon, it provides a unique attack rather than an actual attack, kind of like a dragonborn's breathweapon cannot be used as a standard attack and be used with powers... or so I assume it can't, I could be wrong! For balance reasons, the shotgun is only a 1 shot weapon.
edit: I forgot to mention, I didn't change what they attack, ie swapping attacking AC for reflex, because that could cause potential problems for powers. Some rogue powers target reflex already, so you're complicating that, many poweres are balanced towards their respective defense types. Further, it's a bit of a hold back from previous versions of dnd where firearms ignored armor. I think this is more accurately represented in the higher bonus to hit than it is in ignoring armor altogether.