The "externalities" (CO2, traffic) are less extreme - at least on a per-unit basis, than threatening someone, or actually shooting them. We have the means, if not the will to make some attempts at calculating and taxing them. I wonder what those sorts of calculations would look like if you calculated a value for each person killed by guns in a given year and spread that out over their sales in a tax of some kind. Hrm. Anyway, yeah, cars are useful, guns I can and do live without.
Your comparison is invalid because you're comparing externalities from legal use of cars to externalities from illegal use of guns. Compare the externalities of legal use of cars (pollution, traffic, some crashes) to legal use of guns (pollution from lead and propellants, some accidental shootings) and illegal use of cars (crashes due to DUI, reckless driving, racing) to illegal use of guns (assault, murder).
