samneanderthal

Since the .30 carbine was made only for that rifle, I see no problem in making that rifle with a different case.
Assuming that the military knew what they were doing and there was always a reason for doing things that way is unfounded. As we have mentioned in several instances, there are several factors, including leader ignorance, corruption, etc, that often contributed to inferior weapons.
The grease-gun, P-38, Sherman tanks, B-24, the Airacobra, the M1 carbine, etc, are good examples of basic manufacturing or design mistakes, that the advanced American industry and designers should not have made. They cost a lot to make and performed rather poorly.

The Japanese had a logistics nightmare because they used several cartridges for their rifles and machine guns (rimmed, unrimmed, etc,). Most of their machine guns were extremely impractical. However, starting in 1943 they were extremely efficient building fortifications and defending them. It is surprising how long they resisted and how many American casualties they caused in most islands, despite formidable American air and naval artillery support and overwhelming American troop superiority.