That's not as easy as you may think. Merely rechambering is not sufficient. You have to open up the bolt face too. Further, the Mauser, Oberndorf Commercial 10.75x68 action and magazine must be opened up to work with the longer and fatter .404 cartridge. The 10.75x68 has a CIP max cartridge over all length of 81 mm = 3.19", Mauser mags for this cartridge had an inside length of 84 mm = 3.3". This is insufficient for the .404 with it's max oal of 89.76 mm = 3.53". Mauser .404 magazines on their Magnum actions were 92 mm = 3.62" long.