I didn't read everything here but will give you the short of my experience. I had a mossberg 835 and put a cantelever rifled barrel on it. I couldn't hit a large pizza box at 100 yards, the cantelever would move like a tuning fork bawangggggg.

I went with this http://www.hr1871.com/Firearms/Shotguns/ultraSlug.asp the barrels are extra thick, and the weaver rail is already mounted on the barrle. 3" groups at 100 yards. They don't cost too much either.