Before you take it to bits, try cleaning the parts that mesh when you open/close the gun with a q tip soaked in solvent. Namely, the hinge pin, the hook on the bbls, the knuckle, the matching surface on the forend iron, the water table and the bbl flats, and the action face. The put a little oil on the parts that rub and try again. You will probably feel a big difference. If not, read on.
If the toplever moves freely with the bbls off, there is nothing binding in the bolting mechanism. If it's stiff, you can squirt some WD40 into the bolt raceway (the part that slides forward in the frame to lock the bbls in place) without taking it apart. Try moving the cocking levers (the two pins that protrude through the action knuckle) by hand with mechanism cocked. They should move freely. If not, squirt a little lube into the action alongside them.
Opening and closing the gun without firing it doesn't move anything else so it has to be one of these areas, if it's stiff.
On the other hand, if it's still stiff after you have done this, then it probably needs to be shot in. If it's not, you are almost done but test one more thing: fire it with snap caps or empties, and then open. It will be a little harder to open but if it's very stiff again, then the mechanism needs to be dissassembled to lube the internals as the hammers and springs need some lube.
