I finally got all the parts and pieces to put on my new 870 20 ga. When mounting the FF3 to the B-Square saddle mount it wouldn't fit (clamp to the weaver rail). The grove in the Weaver mount wasn't wide or deep enough for the long threaded stud on the sight to lie in. I had to take a file and make the grove wider and deeper. Once that was done all went well.
I had to do the same thing with mine.