Howdy, Gripmaker!
You have to carefully remove the endcap, not just loosen/remove the screws. The endcap puts pressure on the mag tube which forces it upward closer to the barrel. When you remove the cap, it allows the tube to slightly lower itself which takes the pressure off it.
Careful - the endcap wants to leave marks on the barrel during removal. The Win. 92 is the same, if I 'member right.
Adios!
SHB