The M1 Carbine is the standard by which all PDWs are measured.

[stanc]

True, but kind of explains the concept of drilling the barrel as being some what of an un-proven concept for the time.

Uh, the drilled gas port, gas-operation concept had been proven more than a decade before development of the M1 rifle began. Here are a few examples:

RSC 1918 https://youtu.be/sExBiD7sBw0?t=12
Farquhar-Hill https://youtu.be/oXpgJ4JydN4?t=11
Lewis Gun https://youtu.be/l73mR4D9pYw?t=11

True, but the US and other nations have had a bad habit of manufacturing poorly constructed magazines for firearms, degrading the reliability of an otherwise reliable weapon system.

Did not the Browning automatic rifle and the Thompson submachine gun have reputations for reliable operation?

The en block was probably considered the best alternative, considering the problem Winchester had using a modified BAR magazine for their experimental mag fed Garand.

There's just one problem with that reasoning: Springfield Armory and Remington were able to develop magazine-fed versions of the M1 rifle that worked. https://www.thefirearmblog.com/blog/2015...rock-roll/