Don't get me started on guns as per the DMG. Too much effort went into making sure they were "balanced", if you ask me.
First, they should be simple weapons. Especially the modern ones, but (and correct me if I'm wrong), learning how to load, aim, and fire an early firearm shouldn't be that much harder than learning how to use a crossbow effectively or how to knife fight. They don't need a feat cost to make them uncommon; the gold cost, relative fragility, lack of magical upgrades, noise, and abyssal rate of fire already do that.
Second, they should do a little more damage than a crossbow should, even if it wouldn't "realistically" work that way, it feels more "gunlike" to me, and should speak some to the gun's penetrating power. But it'd probably work best as increasing the gun's damage dice by one step for all firearms and modern weapons. And similarly, increasing the critical rating to 19-20/X3 would also make them slightly more lethal and frightening, without making them too horrid against the PC's. They'd still be pretty much a Wizard's weapon or the armanent of a rich empire, but the mark of a true high-level hero is still his sword. The cost would be prohibitive for most people, as would the loss of attacks for most PC-types.
Making firearms into RTA's also feels "gunnish", but would be an absolute pain to balance, and as such I'll just handwave it away unless the gunner is within 5' of the target. Which makes it absolutely foolish to do if you're in real combat, and not worth worrying about if the opponent is helpless on the grounds that he's already helpless. (The noise should foul up the old "Sleep and CdG" trick, though.)
And finally, if firearms are used, there should be a feat that makes reloading them once a round a free action. Something based on Rapid Reload, probably. Even though it should only require a +1 BAB, since as I said before guns do strike me as a wizard weapon.
So, any opinions/feedback on this?