Try some of these-
http://www.wtw.org/guns/real-reason.htm
"The passage of the National Firearms Act of 1934, the first federal effort to control ownership of firearms, grew out of this historic fear of armed organizations.... In recent times, the federal government has shown itself even less patient with armed groups.... As both history and recent events clearly show, the United States has never tolerated armed groups residing within its borders. The intent of the particular organization, whether ideological or criminal, mattered little.... ATF's enforcement focus retains the flavor of that historic concern with armed organizations."
http://www.op-sec.com/myths/appendix.html AND
http://www.landfield.com/faqs/talk-politics-guns/pro-gun-faq/part2/
"The violence associated with alcohol Prohibition, and the threat of Communist and anarchist subversion during the 1930s, prompted in 1934 the restriction of so-called "gangster weapons" from availability to the general public."
http://www.vpc.org/nrainfo/chapter1.html
"The 1934 hearings stemmed from the wave of violence that had accompanied Prohibition as well as the interstate bank robbery sprees of such criminals as John Dillinger. This, coupled with an assassination attempt on President-elect Franklin Roosevelt in Miami the year before, increased pressure for federal gun controls. Testifying before the House Ways and Means Committee, Attorney General Homer Cummings warned:
There are more people in the underworld today armed with deadly weapons, in fact, twice as many, as there are in the Army and the Navy of the United States combined.... [T]here are at least 500,000 of these people who are warring against society and who are carrying about with them or have available at hand, weapons of the most deadly character."
That's about all I could find on the web. I hope it helps.
Now my $0.02 worth...
The NFA was written, like every other piece of firearms legislation, out of a manufactured fear. Fear was much harder to make enmasse in the 30's. The media consisted of the newspapers and radio. Even after the Depression, people for some reason still had a strong sense of self-worth (and hence a propensity towrads free-will and thought). It took the threat of subversive Communists & anarchists, booze-hound gansters, machinegun-toting bank robbers, heathen assassins, corrupt labor unions and rebelling slaves to stir the pot up enough to get it to boil over. Of course now in the 90's we have killer cults, whacko survivalists & rebellious militias.
I think the equation works like this:
IF GxD>(1/10)US,
THEN PASS LAW
G = no. of people w/guns --> This is measured by 4473's.
D = percentage of the general populous
w/discontent or distrust towards the Gov't ---> Measured by voluntary cooperation with the Census and by increased percentage of ammunition sales following Presidential speeches and Executive Mandates.
US = General Population ---> Measured every 10 years via Census.
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