As Mr. Virginian explained, you aren't supposed to remove what you call the "shroud." It is generally known as the Pachmayr Powerpac Choke System. It was an option you could get on your Model 20 from Sears. There were at least three different chokes that came with the shotgun and others were available from Pachmayr or other manufacturers. However, the threads on some of the other manufacturers chokes weren't the same. You can still find them on GB and Fleabay from about $10 each. You can shot it without choke tubes, but it makes the "shroud" really dirty. The most expensive version Sears offered was with the Pachmayer Powerpac and a ventilated rib, if I remember correctly. It was the first gun my father bought when he came home from World War II. I wish that I had one just to honor him. I'm jealous!