There's a great article on forces involved with firing a load in a SXS action in the NRA Gunsmithing book. Sadly, this book is no longer in print, but available at gun shows and, no doubt used book stores. For a SXS to close, the arc of the barrel breech requires the breech face to form an acute angle to the action flat. That acute angle actually makes the action want to stay closed when cartridge back thrust occurs, even if there was no locking mechanism!

Otto