MacArthur as Chief of Staff of the Army vetoed the adoption of the .276 cartridge on logistical grounds, and probably budgetary ones, remember that was in 1932, military budgets were VERY tight then. A wise call.
J. D. Pedersen spent a lot of time on the grip of his Model 51 to find the most comfortable-OK, ergonomic-shape, IIRC S&W copied it his design with the M-39.