I was able to find some statistics on homicides in South Australia (in
“South Australian Historical Statistics” Vamplew, Richards,Jaensch and
Hancock)

Unfortunately, they only cover the period 1921-1979, but we can use
them to see if gun control, introduced in SA in 1929, had any effect.

We can use the adjacent state of NSW as a “control”. The SA homicide
figures include “manslaughter by driving” and the NSW figures do not,
so they are not strictly comparable, but before 1940 they were only
two or three cases of manslaughter by driving each year in SA.

The second last column is the probability (two-tailed Student t test) that
random variation could cause the difference. The last column is the
Mann-Whitney U statistic (significant at 5% level if it outside the
range 20-70).

   Ave 1921-1929  Ave 1930-1939 Prob chance?    U
SA           3.4            1.5      0.00005    88
NSW          1.5            1.5      0.7        44.5

Conclusion: homicide rates in South Australia fell significantly
following gun control in 1929, while NSW rates did not change.

(BTW, while my source has attendance figures for every cricket game in
Adelaide since 1892, it doesn’t have figures on population broken down
by age.)

The data:

Homicide rates per 100,000 population
      NSW  SA
1921  1.8  3.4
1922  1.6  3.0
1923    1  2.0
1924  0.9  5.1
1925  1.1  3.7
1926  1.6  4.0
1927  1.6  3.0
1928  1.9  2.8
1929  1.7  3.9
1930  1.7  1.2
1931  1.6  2.4
1932  1.3  1.6
1933  1.5  1.1
1934  1.4  1.8
1935  1.8  0.9
1936  1.4  1.9
1937    2  2.5
1938  1.2  1.5
1939  1.3  0.6
1940  1.7  0.8