Yea the M1/M2 carbine had real issues in Korea. The M2(full auto version) was always kind of “iffy” for its entire life. The Marines at Chosin said the carbine just wouldn’t work reliably in the extreme cold. Plus the heavy winter uniforms of the enemy played havoc with penetration. But my reading indicates it was much better liked in southeast asia. Plus the small stature of the people of Vietnam made it a easily handled weapon. I suspect that the fact that jungle fighting Is at pretty close range made the carbine more effective compared to some of the stuff in Korea.
It seems that the M1 carbine is either loved or hated depending upon the experience one had while carrying it. A lot of guys carried it a lot and shot it little and loved it but the guys who had to use it had mixed results.
About the “thumb-buster”….. I have one that I use for cowboy action shooting, It won’t bite you if you hold it right! (grin) And you are right, the .38 spl was originally BP.