changing barrels

[sidhartha]

I have a missmatched shooter that needs a new barrel. I am not sure of the details in changing them though. If I take the old barrel off and screw the new one on will I have to trim the shoulder to line up the sights and extracter grove. Or are they machined in such a way that they are all uniform and the frame and barrel threads start at the same spot. So that when they are put together they will line up with out any machine work. I am sure that I am dreaming here. I used to work in a machine shop so I understand that this would be difficult but not impossible. But then again I am amazed they they were able to do the kind of machining that it takes to produce a Luger for the period in time that hey were made.