I'm a little late to the discussion, but felt compelled to leave some comments.

Venomous Filigree wrote:

So, given that Mythic uses light, medium and heavy damage, how do you think this best fits with firearms? Does this relate to small, medium and large calibre ammunition?

Personally, I think that would be a solid, logical approach. You'd need some additional guidelines, and modifiers for automatic weapons or shotgun 'spread'. You could use 2d6x2 for heavy weapons.

So, starting with small calibre firearms, what's the minimum harm that they might done to a person? If it was equated to 2d6(low dice) I guess even the 'weakest' firearm is going to be + something?

I think this depends on whether you'd use the same 'power level' that's present in BoL. Would you continue to use the Rabble, Toughs, and Villains model to represent the antagonists that PCs face? If so, then even a 2D6L (small calibre) firearm would drop most Rabble and take down Toughs after a few shots (and possibly quicker with leveraged Hero Points).

Armor will likely be less of a factor for most antagonists in firearm-prevalent settings, so you're getting some damage gain there. Except in the case of those who are sporting bulletproof vests or riot gear.