The killing shot / Johnny D. Boggs.

Enhanced descriptions from Syndetics:

Deputy U.S. Marshal Reilly McGilvern is hauling criminals to Yuma when his prison wagon is attacked. Three guards die in a hail of gunfire and McGilvern is left locked inside to die. When another outlaw gang comes upon the scene, Reilly McGilvern thinks he'll live to see another day. What he doesn't realize is that his problems are just beginning. Bloody Jim Pardo wants to avenge the Civil War and needs to steal the kinds of weapons to help him do it. Riding with his mother, his thugs, and two hostages, Pardo mistakes McGilvern for a fearsome criminal. To stop Pardo from carrying out his mad caper, McGilvern must play his part perfectly - and when the time comes, every shot he fires must be a killing shot. Winner of the Western Heritage Wrangler and four Spur Awards for his Western stories, Johnny Boggs is one of America's great Western writers. He combines adventure and realism with a vivid storytelling style all his own.