RE: 11-87 Barrel
Yes, you can use that barrel. The 11-87 Super Mag comes with a smooth bore barrel that's chambered for 2 3/4" - 3 1/2" shells. It's just the barrel that's chambered for those shells, not the receiver. If you buy the fully rifled slug barrel that's chambered for 3" shells, it will fit just fine but you'll only be able to use up to 3" shells.