Pat is only partly correct here. H&H called the .400/.375 the ".375 H&H", other than the ".375 H&H MAGNUM". Hence the MAGNUM monicker attached to the longer and much more powerfull 1912 vintage cartridge. Look at page 52 of the 1910 catalog! There is simply no way to lengthen any M-S magazine and receiver to take the long .375 H&H Magnum or the .404. As I wrote above, even the post-1952 "Magnum" actions are limited to a cartridge over all length of 86.5 mm, the pre-war M1925 ones to 85 mm maximum. The .375 H&H Magnum has a max col of 91.44 mm, The .404 of 89.66 mm.