I used to think that the j spring of the snap on forend was what kept the forend snug to the frame, until I fitted an orphan set of barrels to an LC Smith that I own. I was able to fit the barrels to the frame in a few minutes, but fitting the forend to the frame and barrels took a few hours, because the length of the slot in the FE iron is fitted to the FE lug to control the friction between the FE iron and frame knuckle. If the slot is too short, the FE won't go on and if the slot is too long, the spring causes binding and excessive wear and galling. I solved the problem by installing a headless set screw in the front of the barrel lug to adjust the clearance between the FE iron and the lug on the barrel.
