I think Gourd’s remark about the .30-06 being used in the 7.62 x 54R caliber rifles hinges on the phrase " most guys were ignorant of small arms" and on the double exclamation mark at the end. In short, they believed something that was totally untrue.
A lot of the American Korean War troops had funny ideas about the “one-way interchangeability” of Communist ammo with our own. I believe it started with the real case of small morters of Chinese or Russian design and manufacture being able to drop our 60mm rounds but American forces could not drop the slightly larger diameter communist rounds in American mortars.
In their minds, they expanded that out to a lot of different things that simply were not true.
Gourd can correct me if I am wrong. I wasn’t there, of course, but did serve, beginning about three years after Korea, with dozens of Korean War vets, and I was as much of a gun person then as now, and pounded them with questions. Some of the answers I got astonished me even then, when I didn’t know 1/100th of what I do now (I don’t know much now, but of course, then, I was 18 years old and thought I knew everything there was about guns and ammunition, because after all, I could take apart an M1 Garand blind-folded, and load it without ever catching my thumb). Some of the communist weapons info those vets had was pretty far off the mark. But they sure knew our own weapons, and I wouldn’t have wanted to be on the other end of the muzzle from any of those guys!
I would bet that ammunition in used by the CCF in Korea in M1 Carbines was 95% or more WWII U.S. given to China during that war against the Japanese. And yes, the Chinese had plenty of M1 Carbines, Tommy Guns, and other U.S. Weapons in Korea. I am not sure the North Koreans had so many, however. They probably had some, but I would guess they probably had a lot more Japanese weapons, and of course, Russian ones.