There is a certain gent in Oklahoma I knew in 2003/4 who had brought back a Kalashnikov from Viet Nam hidden in equipment. One of those really good chaps - he invited me to his farm for a whole afternoon and we had a good time, even though we’d never met before then (it was a church thing). I am sure there is a lot of ‘stuff’ that came back after Viet Nam inside speakers and other equipment. The gun was full auto, but this is in Oklahoma (where people tend to use logic in deciding whether to report someone to the authorities), and thus he has had it for decades now. It is also in a farm, surrounded by friends, and people tend to mind their own business unless one becomes a bother. I am still surprised at how welcoming the guy was. To this day. Its not every day one gets treated as family by a total stranger. Generally, my time in Oklahoma was marked by almost constant surprise at the level of honesty, grace and warmness by the people. Absolutely amazing. Comparing that with, for example, Minnesota, showed a stark difference (maybe it is the cold weather in Minnesota that makes people grumpy lol). Oklahoma will always be a second home (and that is the end of that attempted thread hijack). On the guns ...a lot of stuff came via speakers and stuff.
I heard from the USMC grapevine of a guy getting a complete M-60 back. Shipped in what, no idea, but that sucker is not light. Again, what do you do with it? Assuming you can find linked ammo, would be easy to blow a month's pay doing an afternoon plinking session.
As others have said, lots of stuff came back openly, but as I recall, all were semi auto SKS and bolt action stuff, no full auto AKs.