At least in this state, the property tax is due regardless of where you use your car. All using a motor vehicle only off the public roads saves you is the annual license fee. The property tax is general revenue, the license pays for highways. By analogy, having gun owners pay for the costs of investigating gun crimes, ER and other medical costs of gunshot victims, etc. seems quite reasonable.
Not sure the analogy holds. It's a reasonable expectation that car owners are also users of highways, and the overlap is likely quite high. It is not a similarly reasonable expectation that gun owners are consumers (or causers, if you want to put it like that) of those services.