As far as scoped 870 goes, I have been happy with my cantilever scope rail. You all are right that the zero will not hold on a normal 870 with it mounted on the receiver.
Since I shoot $3 a shot sabots I thinking of building a dedicated sabot gun.
I may use my 870, but I am going to pin the barrel to the reciever, that wayeach shot will be more consitant, the jiggling barrel is a huge disadvantage in accuracy. Then I will be able to get a nice rail mount and optic for it.
Remington is also offering an 870 variant that come with a pinned barrel. I would love to hear how they are doing accuracy wise.