On the Ruger, the barrel is screwed into the receiver and can be removed. But Ruger considers the barrel-receiver as a permanent assembly and does not sell barrels separately. Note I said "receiver". This is the tublar part behind the barrel in which the bolt rides. The part with the handle is the grip frame, not the receiver.
If you want a different barrel length, Ruger will do the work. Call or write them.
As for serial number location, there is some precedent on the Luger, where BATF considers the receiver number as the serial number because the barrel-receiver unit can be fired without the grip frame. This is not true on the Ruger, but Ruger serial numbers the receiver anyway, mainly because numbering the light steel grip piece would create problems.
[This message has been edited by Jim Keenan (edited April 29, 2000).]