Yes, I'd hoped for a little more elaboration after that too.
I'm guessing that at least some of the mass killings of the '20s were caused by anarchist bombings. Probably a few Prohibition-related mob shootings in there, too. (St. Valentine's Day Massacre was in 1929.)
Also we have recently been reminded that the worst school mass murder in US history was committed in 1927, without guns.
The 1929 date seems quite suspect, given the chart accompaning this article by the researcher, Grant Duwe: http://www.aolnews.com/2010/03/01/opinion-the-rise-and-decline-of-mass-shootings/
See my post #31