HERE is a Remington 11-87 Sportsman 20 ga. with a 21 inch fully rifled barrel with the cantilever scope mount.
My father wanted to go with a 20 ga. so he bought me the 11-87 Sportsman 12 ga. and I gave him my 11-87 Premier 20 ga. Remington makes a 21 inch fully rifled barrel with rifle sights for it. It has the blued finish to match the Premier finish. The Sportsman 21 inch fully rifled barrel with the cantilever scope mount has the matte finish and will also fit the Premier model. He's going to use a scope so we just bought one of the Sportsman barrels at Bass Pro Shops this morning. I put it on the gun when we got home. This should make a nice lightweight slug gun. After we make sure it shoots good my father is going to have the blued.
Remington slug barrels for the 11-87 20 ga.
PT# 29563 Premier Deer (blued finish) 21" fully rifled with Rifle Sights (2 3/4" chamber only).
PT# 27591 Sportsman Deer (matte finish) 21" fully rifled with cantilever scope mount. (2 3/4" and 3" chamber).
Hastings makes a few barrels for the 1100 20 ga. as well.