In Boston from 2010 to 2015, there were 221 gun homicides. Research suggests that one change could have lowered that number by 40 percent: smaller bullets. A study last year, published in JAMA Network Open, examined the type of weapon used in every fatal and nonfatal shooting in the city. It found that — regardless of the time of day, the number of wounds or the circumstances of the crime — the size of the bullet affected which gunshot victims lived and which ones died.