An otherwise complete AK-47 rifle without the firing pin remains a machinegun under federal law (readily restorable to functional condition), and anyone possessing such an item or attempting to import it into the United States is committing a serious felony offense. Even the rifle's receiver completely stripped of all other parts and firing controls remains a machinegun under federal law.Yes, he could bring it home, but it didn't have the firing pin.
US military command authorities were very well aware of these laws and strictly enforced compliance. I have no doubt that a number of restricted items were smuggled back to the US, but not with any type of official sanction or legal exemption. Since most of the troops were young men with limited experience (average age of infantrymen was 22) I'm sure there were any number of misconceptions or misunderstandings, but I seriously doubt that any command officers or senior staff ever allowed any soldier to carry a AK-47 out of Vietnam. Frankly, any officer or NCO who knowingly allowed such a thing to happen would also face court martial proceedings, probably lucky to get by with a Bad Conduct Discharge in lieu of a prison sentence.
Good stories are common, especially around poker tables or pitchers of beer. Even more common are the "I knew a guy who knew another guy who swore that he heard some other guy ..............". Hang around any group of veterans for a while and you will find that no one was a truck driver, or a supply clerk, or a cook, everyone was Special Forces, or SEALs, or Recon Marines, or fighter pilots.
formerly Sergeant, 11F4P (infantry operations & intelligence, airborne, Pathfinders), Vietnam 1969-71.
formerly US Army Reserve Training Command.
retired Police Chief.