Actually the Darne is an extremely strong action. When the toggle closes a bolt rises and fits into an extension between the barrels. The toggle cams "over the top" making for another lock up. Darne uses this same action for rifles,in some of the older large dangerous game calibers. My personal Darne is a little 28Ga straight grip that weighs in just under 5 1/2 pounds and is a delight to carry and shoot.
There are many Darne knock off's, some of dubious quality, some good and one in particular that is not a true knock off but a design all of it's own, although af first glance it looks like a Darne. That is a Charlin, and is recognized as a weaker action, locking only by the toggle camming over. What sets it apart is that the sliding breech rides on little hidden ball bearings making it super smooth. Mine is not one of the highest grades, but has minimal engraveing, and what is amazing is that at the instant the toggle locks down the engraveing all lines up and matches.