In the 1920s and early 1930s, you could buy pretty much anything legally, by mail order. The Sears Catalogue in 1929 advertised the Tommy Gun (“Anti-Bandit Thompson Sub Machine Gun”) for $175, (a little over $2000 today, though relative earnings differences make it closer to $5,000 in today’s earning equivalency) a 50-round drum magazine for $21, far more than the standard 20-round magazines of $3.00 each. http://img.photobucket.com/albums/v613/Tim_Orrock/thompson_stoeger600.jpg But I’m sure Capone could fork out the extra dollars for the magazine, the $25 for a compensator (makes the gun easier to handle when firing a burst) and so on.

There were some regulations. Looking at the advertisement for the Boys anti-tank rifle from the late 1930s ($98.50), California residents needed to add 3% sales tax. (Ouch. I live in California. I’d almost I’ll for that level of sales tax again)

Of course, this was deemed to be a little out of hand, and this whole gangster machine gunning people business was the cause of the 1934 National Firearms Act, which implemented a whole slew of regulations including restricting (not banning, just adding hurdles) the sales of machine guns, which the law defined (and still defines) as weapons which fire more than one round per movement of the trigger. Once that happened, then buying Tommy Guns and BARs became a little more difficult (not impossible, of course), but by that point Capone’s gang had likely had sufficient stocks to last them a while. However, the act was arguably a success. Only two murders since 1934 are known to have been committed with a legally-owned machinegun since the Act was passed, one in 1988 and one in 1992.