Looking at the powder loads, velocities achieved, bullet diameter, mass / energy of the calibers. My guess would be that the .30 fell short in your tests due to the particular type of bullets in the rounds tested. Just saying "FMJ" doesn't begin to tell the whole story of a bullet. Most people would be amazed at the research Russia did before releasing 7.62x39, testing and comparing lead densities, thickness of the brass cases, grades of brass used, where the weight of the lead in the bullet would be / how thick the brass was at the base, nose, and sides of the bullet. The 7.62 looks like poop on paper. But consistantly impresses in the real world because this research. An EXCELLENT book IMO on this kind of stuff is "Modern Handloading". Or "The Complete Book of Practical Handloading" isn't bad.
Again, bummer the .30 didn't pass those particular tests, but there are several manufactures out there for the caliber today, and all kinds of handloading supplies for it.
IMO... The US did little to no research on bullet construction back then (so surplus ammo would still be crap). History states clearly they were in a rush to get into production with it. But a .30 Carbine with modern ammo would be no kind of disadvantage over anything else for the average citizen.