Remington 870 Super-Mag Forcing Cone
The 870 Super-Mag has a 3.5 inch chamber. I think what they did was take a 3 inch chamber and ream it out to hold a longer shell. It looks like a lengthened forcing cone, but it is very rough. I sent mine to a guy named Johnny Stone in Oklahoma..He polished the forcing cone, and put a middle bead on the rib for me.
I also got some Trulock extended choke tubes. It's a pretty nice outfit now for how much I have in it.