Seeing all these precious examples of experimentation with the carbine as it progressed to the final issued product is very interesting. It seems they must have tried about everything on anything before adopting the carbine as we know it today. If any one of the changes made had actually stuck in the final design, that design would be the carbine we luv and cherish today. Dual springs, various sights, different gas systems, stamped metal stocks and the list goes on.
Other calibers are certainly suited to perform in the carbine action and I have tried a few. In the end, the caliber and the design that was chosen was most certainly the best one.