I thought I had seen a Johnson Automatics barrel on a 1917 that was among some rifles captured in Korea from the PLA. Most I have seen were on O3A4s tho. Never seen one on a '03, but I suspect there are some around.

People sometimes get the 1941 semiauto rifle mixed up with the Johnson LMG, which was also USMC issue, and probably what turned up in "Cuber," as JFK called it.