“Once we had that, as simple and obvious as it sounds, then we started building devices that could swing a man over a continued distance using that principle,” Andy Armstrong says. Stunts are something of the family business for the Armstrongs. Older brother Vic first got into the industry in the ’60s as a steeplechase jockey and accomplished equestrian who fell into helping out with horses on film and TV sets. With his riding skills and daredevil nature, he quickly became an in-demand stuntman. Now 65, he prefers directing the action rather than being in it. He recounts some of his greatest tales in “The True Adventures of the World’s Greatest Stuntman,” issued in a paperback edition this month.