There is an individual (one man operation) who makes 408 bullets for EDM (not sure of the weight). Also, FWIW one of the major bullet companies is working on a 408 bullet (not a solid).
The 375x408 is much more versatile cartridge, especially with the available bullets. Sierra's 350 SMK shoots really good
in the 375x408 Chey-Tac (Lawton Action/Barrel in a MCree stock)