Went back to the range today with the same 870/cyl. barrel and another Mossberg. #1 Buck FC worked normal with the Mossberg. With the 870, one hole at 25 yards. Moved the target out to 50 yards, finally a pattern of 3+ inches. The wad seems to be staying with the shot past 25 yards, then separating.
That barrel started life as a 20" imp. cyl. with rifle sights. Sent the shotgun/barrel combo off to Wilson combat for their "Remington Steal" package and had them cut the barrel back to 18" and install a bead on pedestal. Don't know if that has anything to do with it. Normal buckshot patterns as you would expect, 1" spread for every yard, 14-15" at 15 yards.