Mighty big leap of logic there...but no, that's not it (at least for me). I just want a player who chooses a gun over a crossbow to have actually made a choice. This is the problem I have with reskinning things or just changing fluff... all I've done is give the player the illusion his choice actually matters in the game... especially when it comes to combat (which in 4e is very detailed oriented). I mean a gun might do more damage than a crossbow, but take longer to reload... or have a chance to misfire... or even have a chance to explode and hurt the character in exchange for that greater damage.
And, right there, you've nailed why there's no gunpowder weapons in the DMG. Why simplistic, half baked rules don't work. "Backfire" mechanics have never worked as a balancing mechanic in D&D- players either find a work around or just choose something else because the backfire mechanic is too much of a PITA.
Again, it goes back to an earlier post about realism. ((Sorry, forgot who posted and too lazy to track back)) We don't worry about upkeep on bows or crossbows. How many new bowstrings has your elf ranger EVER bought? How long does it take to string a bow? How long can you keep a crossbow cocked before the string stretches?
Who cares? We hand wave all that for the same reason we hand wave all that stuff. It's boring and adds pretty much nothing to the game.
So, if you want actual rules for gunpowder weapons in D&D, that players will actually use at the table, you need to go a bit beyond this sort of thinking. There's been some great 3e gunpowder rules out there, none of them core.
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On the best DMG.
I don't own a 3.5 DMG, so I cannot comment. But the 4e DMG is light years ahead of the 3.0 DMG. It's actually a book that teaches players how to DM, rather than a random collection of rules and minutia that pretty much never gets used.
I mean, other than the magic section, maybe the traps section, possibly the demographics section (depending on your campaign), how often have you read the DMG? If the conditions list was in the PHB, I don't think I'd ever open my 3.0 DMG after the first time.
I'm more than willing to stack the 4e DMG against the 1e DMG in terms of utility and actually teaching people how to DM.