Claim: The US 75 mm gun, which was the standard American gun mounted to a mobile platform during World War II and came with four variants M2, M3, M5, and M6, have different muzzle velocity, 75mm L/31 (M2)  has only 563 m/s (1,850 ft/s), while 75mm L/40 (M3/M6) has 618 m/s (2,030 ft/s).
Evidence: The US 75 mm gun was the standard American gun mounted to a mobile platform during World War II. There were four variants used during the war: M2, M3, M5, and M6. 75mm L/31 (M2) 563 m/s (1,850 ft/s) 75mm L/40 (M3/M6) 618 m/s (2,030 ft/s)