There were probably over a million of NFA items in the country, when it was passed, considering it covered not only machineguns, and silencers, but also all short barrel rifles & shotguns (below the listed length) and stocked (or capable of taking a stock) pistols as well.
Did you know that an original draft of the NFA did not contain regulations on silencers? It had the NFA regs for HANDGUNS! That was dropped, and silencers substituted, because (some) cooler heads knew including handguns was going too far (even for them), and would result in the bill being killed in Congress.
The 1930s were an interesting decade, so many advances in so many good things, balanced with the "advances" in human misery. The rise of dictatorships (or near dictatatorships) in many countries, and the laws that resulted, some of which still remain with us.
All else being equal (and it almost never is) bigger bullets tend to work better.