There is a company in the US -lord knows I can't remember their name but they are in Idaho, that makes threaded stubs for contenders. These have the extractor cuts and locking lug cuts already made. It then becomes an easy chore for a gunsmith to fit a straight threaded barrel to the stub and headspace it. The extractor cuts and assembly would finish it. How radical a change of calibers will dictate what parts, if any, will be needed to complete the job.~Muir
FWIW: I am doing one the "hard way" by boring out a 30-30 barrel and threading a 25 caliber blank to fit so I can have a .256 Winchester Magnum. Mine is a GI contender, tho.