I suggest you check and then read and re-read the barrel fitting section in the FAQ's. After all that it may just fit fine when you get it. An inexpensive used shotgun barrel at about $40 would be just the ticket to start with to try your hand at fitting a barrel....<><....
(try it on the frame, it will either close and lock in position or not, then when it is closed check to see if the back of the barrel by the chamber is close to the action That is where you can start...IF you have to fit it, go slow and do only a little at a time, it is easier to take metal off than to put it on, but IF you make a mistake it is always pretty easy to shim up the barrel to fit...)