FWIW, QuickLoad lists (PSI) ratings for the various cartridges in question thusly:

.303 British = (SAAMI) 49,000; (CIP) 52,939
.375x2.5" = 31908
.348 Winchester = 46412

The .45-70 shows (SAAMI) at 28,000 and (CIP) 31,908. The .444 Marlin shows at 51,488. The .375/.303 shows at 45,991.