I just picked up a 6920 and a standard ARMS flip up BUIS. I cannot for the life of me figure out how to put the BUIS on the rifle. The flattop is just too wide for the sight to fit over. Do I need a special mount to mount the BUIS or did I get the wrong model BUIS? I believe it is the ARMS #40 that I have.

I pulled back the charging handle and the sight fits over the very back of the top rail. Where it gets stuck is where the rail itself starts. At the rear, you have the rounded off part and then the part that the "hooks" of the sight fit under. The rail is too wide for those hooks on the sight. I cannot get the bar of the sight into the first slot of the sight rail. I wish I could provide pictures but I do not have a digital camera. Somebody please help!