I thought I should expand a bit on the comment in the high tech weapons post about every century adding a bonus to the effective damage dice. I'm basically assuming that, once a given technology gets past the unique or "weird science" stage and become more common in a culture, that culture becomes better and better at making that weapon, in much the same way that a person with 20 years of experience in a profession will generally be better than someone who has just started. It's a crude approach to tech level. Guns, for example, should get lighter and sturdier as time goes on. Since I'm linking the damage dice to the size of the weapon, I assumed that the maximum size of black powder weapon that can still be used by one human being, acting alone, drops every century.
Or, to put it another way: The number of centuries since the introduction of guns, plus one, equals the damage dice of a two-handed personal firearm. (I say "two-handed" because the pictures of a medieval handgonne seem to suggest it was more like a bazooka or mortar rather than a pistol.) That's the damage rating for a weapon usable by a creature in the 1 to 4 HD size range; every extra die of damage adds 4 HD to the minimum creature size.
The exact number of years is arbitrary; I picked a century because it seemed slow enough. I could drop the reference to time entirely and just say "this is an advanced 3 dice two-hand firearm" instead of saying it's from 200 years in the future.
You can trade one die of damage to halve the weight, or 2 dice to make a two-hand weapon into a one-hand weapon. You can also trade a die of damage to double the rate of fire or improve the range. I haven't decided how much the range should improve yet; I'm not big into emulating specific historic firearms, I just want a quick way to represent a mix of weapons.