Writers are normally Professors of Puff instead of really knowledgeable of the mechanics of firearms. If you want to learn about doubles, read this site for a few years. Now Macintosh has a great relationship with David Trevallion and with David's knowledge and Mac's writing skills, they make probably the best writing team in all gundom. That shows they are both smart! Any article or book with them together is a treat, eminently readable, and eminently accurate.