Absolutatootly, in D&D (from which most d20 systems have some relation to) the basic mechanic is exactly as you described in comic. For something more *explosive* you just need the right supplements (Heroes of Battle provide some sample siege weapons, but no cannons. Personally I would check out the Pathfinder prd as they have quite the selection of firearms and siege firearms.) and the proper in-game research.

Hehe, hell. You could make a muscle wizard do that with his fist so doing it with a canon shouldn't be a problem. ^_^

As I understand it, a weapon has to be at least +1 to have any enhancements, but enhancements from weapons & ammo stack... not sure if the pluses to attack & damage stack or not

nah, a +1 bow using +1 arrows still only gives a +1 to your attack and damage