While the M1 was general issue later in the war, some units carried the 1903 Sprinfgield from Pearl Harbor to VJ day (and not just in the Pacific). Some units in the Italian campaign were still entirely equipped with Springfields at the end of the war.
My friend said all the ammo he got in Korea was AP (carried a Garand then).
Another friend of mine was a telephone/telegraph lineman in Korea. He carried an M1 carbine and loved the light weight. He hated the winter there, because they took away his carbine and made him carry a Garand. He also made it a point to explain he never had to shoot anyone, and if he had, his opinion might be different.
All else being equal (and it almost never is) bigger bullets tend to work better.