Seems like a good start, although I'm concerned that double barrel length = double damage/double throw could be a bit overkill. I'm currently looking into GURPS Lite and how it suggests to model firearms (I still like the idea of only using d6 for everything). http://www.sjgames.com/gurps/lite/3e/gurpslite.pdf On page 20. Seems like a pretty good start, and I think one can easily define some sort of modification table for certain calibers and barrel lengths and so on. 9 mm -> 2d6 base damage or something like that, other modifications allowing +2, x0.5, whatever sorts of modifiers. Looking at the tables the amount of dice can become quite daunting though. Blaster rifle doing 12d6 damage? But that's subject to change, of course.