I have that set up on my Wilson Combat and it is flat out awesome. I am waiting for my FN shotgun to be delivered from the Duracoaters, but when it arrives, I'd like to put the light on my FN. If you find out whether it fits or does not fit, I appreciate being told the answer.
I have one of these forearm lights on my BENELLI M1 SUPER 90. But I don't have a FN shotgun so I'm not sure if it will fit that model. The only problem that you might have putting it on the FN is the spacer rings. Why not contact Surefire and ask them? I've contacted them before and they are very helplfull.