Short answer:
The ribs are taken off and the worn out or damaged barrels are cut off to about 3" stubs at the chamber ends. The stubs are bored out and new full length barrels are soldered in place in the stubs. The ribs are re-attached and the barrels are finished.
This is a very simplified answer but hopefully it makes the point.
Jim


The 2nd Amendment IS an unalienable right.