Why? Actuaries for life expectancy, and the likelihood of a fatal shooting should be fairly accurate IMO. Fatal accidents due to head injury are difficult to find, granted, but you can get in the ballbark. I don't understand why this keeps it from getting off the ground.
There was a related math problem recently in Australia, a little simpler because they only concerned themselves with actual costs. Cost of helmets vs cost of medical procedures. Their surprising result was that it cost more overall to require helmets than they saved in medical costs.