There are several threads on the CMP forums about accurizing the M1 Carbine. As I recall the fit of the stock at the barrel and barrel band are pretty critical. You need some up pressure at the tip of the stock. Also, even though many people don’t like them from an historical perspective, the post WWII barrel band with the bayonet lug helps with accuracy. My ‘43 Inland, which most likely has its original barrel, came with a generic wood stock. I was able to half it’s grouping size by changing to a 1945 RIA replacement stock.

Mine likes Remington factory .30 Carbine. It seems to be a bit slower than the 1960s US surplus ball I found and shot, but is more accurate.