I carried one for a week or two while I was in Vietnam. Looked great and was simple to clean - but I realized very quickly that it was completely ineffective past maybe 50 meters and with a loaded magazine weighed exactly the same as my loaded M-14.
Since my M-14 was much more effective at all ranges, I ditched my M3A1.
“I carried one for a week or two while I was in Vietnam.”
I used to see Montagnards carrying those. I always wondered where they got them.