"While it entirely possible that between one and two million fully automatic weapons were in private hands in the United States at the time of the passage of the 1934 act, that would have required that practically the entire production of German automatic weapons to have been transported home as souvenirs"
I'm sure most of that number were domestically produced weapons. Yes in 1918 fully automatic weapons were probably still very much a novelty but by the 1920s and early 30s you could buy a Thompson or similar weapon over the counter for less than $100. Wikipedia claims for example the Browning Automatic Rifle was a popular civilian weapon
I also read that within the first few years after of the act's passage only a few thousand weapons were registered meaning most gun owners did not comply with the act and is was difficult to enforce.