I decided to check the claim that crime went up following gun control
in England in 1920. I compared the homicide rates for England for the
period 1911-1920 with the period 1921-1930 and found that the rate
declined by 8%. This change could have been caused by demographic
changes that followed from World War I, since this killed a
significant number of young males (the group that murders most.)
To check for this, I looked at all the countries in  for which
homicide data was available covering 1911-1930. A feature common to
almost all these countries (Japan being the major exception) was a
pronounced dip in the homicide rate in the years 1915-1918,
presumably because large numbers of young males were serving in the
war, so I excluded these years from the analysis, to get a better idea
of the true homicide rate before 1920. We also want to establish if
any changes are significant, so I did a t-test for each country.
Here are the results:
11-14 21-30 change Prob chance could 19-20 % cause this change England 0.80 0.69 -12 0.003 Australia 0.89 0.67 -25 0.05 Germany 0.58 0.84 +41 0.001 Italy 7.85 10.2 +29 0.24 Japan 3.46 3.78 +9 0.14 Portugal 2.82 3.16 +12 0.17 Scotland 0.74 0.78 +5 0.55 USA 7.52 8.58 +14 0.006
(Note that some of the rates are for homicide offences and some for
convictions, so you cannot compare rates across countries.)
We see that in most countries the homicide rate increased, with
Germany and the US recording significant increases.
The only country beside England to record a significant decrease after
1920 was Australia. NSW (40% of Australia's population) introduced
gun control in 1920. Victoria (30%) followed in 1921, and Queensland
(15%) in 1927.