How would small arms develop if the US adopted an intermediate intermediate cartridge (between assault rifle and battle rifle caliber) in the 1920s-30s? The way I understand it, the military started realizing that the M855 5.56mm round just wasn't enough, and evaluated 6.5 and 6.8mm cartridges throughout the 2000s. They also weren't alone in the 20s and 30s either, since the Japanese had their 6.5mm Arisaka.