.30 carbine's one of those funny "intermediate" calibers. Basically similar in energy to .357 magnum, but with a lighter bullet and higher muzzle velocity which actually makes it capable of defeating most soft body armor, like other "rifle class" cartridges.

The select-fire M2/M3 carbine is America's first true "assault rifle" in that regard, and we had one fielded in limited numbers in 1945, shortly after the germans had introduced the MP/StG-44, and way before the russians fielded the AK47. However, the commies went on to equip their entire front-line force with assault rifles, which the US didn't do until the widespread introduction of the M16 in the 'nam.