Your figure is off: according to the center for gun policy its 270-310M guns. 110M rifles, 86M shotguns and 114M handguns (roughly estimated). There are 253M cars.

It's also worth noting a number of mass-shooters, including the Virginia Tech shooter legally purchased the weapon AFTER being diagnosed with a mental disorder. A method to prevent this from occurring would logically reduce or encumber mass-shooters sufficiently to lead to their arrest or deterrence from committing the crime.

Enforcement of existing laws and some additional common sense things (like requiring all sales including person to person go through FFLs and backgrounds) would help reduce the incidents of gun crime.

We don't need to ban guns, but we do need to behave smarter about them and realize that unrestricted 2nd amendment rights just aren't realistic nor would the framers endorse them if they could see the weapons of the modern world.