Treating gunshot wounds costs the American taxpayer almost a billion dollars a year, according to a group of University of California researchers. They looked at the hospital records of all 131 patients admitted to San Francisco General Hospital with gunshot wounds in 1984. As reported in the Journal of the American Medical Assn., the researchers figured hospital costs totaled $6,915 per patient. Extrapolating the San Francisco figures to the United States as a whole, which has about 62,000 firearm injuries annually, and adding other medical costs--such as ambulance fees, physical therapy, doctors’ payments and long-term care--the study’s chief author, Dr. Michael Martin, said he “conservatively” estimated nationwide costs to exceed $1 billion annually. Government sources, such as Medicare and state medical assistance programs, covered 85.6% of those costs.