In Accident Reconstruction of Low Speed Impacts, accident reconstruction and automotive engineering expert witness writes:
Even if high speed barrier crash tests could be validly generalized to low-speed crashes with moving cars, they cannot help predict the energy transferred in any one crash because vehicles differ considerably in their ability to absorb certain impacts without showing damage. In this case, that variability would apply to the damage to the bullet’s front bumper and target’s rear bumper.
One example of the variability between vehicles is that older vehicles generally have stronger bumpers and can absorb greater impacts while showing less damage than newer cars because newer cars are frequently equipped with bumpers where style dominates over function.