I had posted Graham Greener's explanation on the strength of the Greener Facile Princeps action here some time ago though I could not locate the thread just now. It was considered the strongest break open double rifle action in the golden days of British gunmaking before WW-2 broke out. Some consider it ugly and the problem that Greener faced was that it was extremely expensive to manufacture and they lost a lot of money making them becaue no one wanted to pay as much for a boxlock design (which is what the Facile Princeps was in the end) as they would pay for a sidelock. Greener recently reintroduced the design and have made the first Facile Princeps actioned double rifle (a 375 Magnum, I think) since the war but I have also heard that they are concentrating more on sidelock rifles as they seem to be facing the same prejudiced that they faced in the past. Sad, and I hope that the design does not go into cold storage as it did post WW-2.