Well, the breech end of the barrel. The 1.1x12 Mauser threads cannot really be remachined to the Remington 1 1/6-16 threads. Better to cut the threaded portion off. The breech of the barrel has to have enough steel to allow this shortening, and still have enough material to cut the Remington breech. It depends on how the barrel breech is contoured. Apart from the chamber being partially cut, it is about the same amount of work as starting with a tapered blank.