The problem with conventional armour-plating is that metal is very heavy and is a big waste of fuel when the car is being driven when the occupants are not at risk.
Researchers at Norway's Forsvarets Institute have found a way around this by building several large, flat, watertight tanks into the sides of car, New Scientist reports.
Each tank is thin, like a domestic radiator, made from plastic or light metal, and has several energy-absorbing carbon-fibre sheets stacked inside.
When the tank is empty, it offers no protection against bullets, but adds little weight.
But if the tank is filled before a risky journey, the combination of sheets and water provides a superb shield.
The device has been tested against high velocity rounds and has now been patented, the British weekly reports in next Saturday's issue.