Remington made 20 gauge shotguns on two frame sizes. Many parts (including barrels) are not interchangeable between the two size frames. There seems to be much confusion about determining the correct frame size especially when purchasing a slug barrel. Here is how to tell them apart:
Frames that have a serial number that ends in “K” or “U” are the small frame guns. They have 20 gauge diameter magazine tubes and barrel rings. These are true 20 gauge size guns. They are lighter and slimmer than the large frame guns.
Frames that have serial numbers that end in “X” or “N” are large frame guns. They have 12 gauge diameter magazine tubes and barrel rings and are built on 12 gauge size receivers. They require 12 gauge trigger groups and 12 gauge wood. They are larger and heavier than the small frame gun.