Here's something completely unrelated to the posts over the last few days: handling high-tech weapons. I've written before that I prefer using 1d6 for weapon damage for all mêlée and basic missile weapons, instead of varying the number or size of damage dice based on perceived deadliness. Instead of increasing damage dice, I'd add a +1 or +2 to hit with some weapons and allow a couple other special effects, such as making a heavy crossbow (or any heavy weapon) treat heavy armor (plate) as medium armor (mail.) The only ancient weapons that do increased damage the way I run the game would be siege weapons and weapons built for use by ogres or giants.
And yes, I'd want to just treat a medieval handgonne as a kind of heavy crossbow and cannon as a ballista. But presumably there would be larger guns, especially if later technology makes it into a medievalesque gameworld. I think my simple rule will be that yes, larger gunpowder weapons would do more damage: they do the same damage as an ogre or giant, depending on how big the gun user would have to be to lift and aim the gun. As technology improves, "big" guns can be made smaller; figure that every 100 years of experience an advanced culture has making guns, a gun does 1d6 more damage based on its size; however, if a gun does more damage than the gunman can deal out with a mêlée weapon, any damage die that comes up as 5 or 6 means the recoil is too much for the shooter (weak characters will be knocked down, stronger characters have a penalty on the next attack.)
Guns with a higher rate of fire do not increase damage; they increase the chance to hit. Or, if they fire fast enough, they act like area effect weapons (Lightning Bolt.) However, as Cracked recently pointed out, machine guns can only fire at their maximum rate for one minute before the barrel needs to be replaced.
Energy weapons eliminate the recoil, but do have a chance of draining their power source or possibly even permanently burning out if fired at max damage. This all seems simple enough that I can handle it without page-turning.