Another cause of a loose forend lug is lack of or an improperly fitted barrel check. The lack is most apt to be found on a hammer gun. I have an H Pieper Modified Diana hammer gun which has a loose lug & section of under rib which is no doubt from this cause. It was like that when I acquired it, very cheaply with intentions of practising a bit of re-laying, just never got to it.
Anyway the only thing which stops the rotation of the barrels on this gun is the rear bottom of the forend iron contacting the bottom of the frame bar. If you apply the least amount of force here tremendous forward pressure is placed on the forend lug, far more than could ever be done by gripping the forend against recoil Even with a barrel check if the barrel motion is not halted prior to the forend iron contacting the frame the check is of no value, its just along for the ride.
