Obtaining a satisfactory proxy for the motivation (M) to purchase assault rifles is a difficult task.
In the first instance, income growth is adopted as a measure for the desire to buy weapons.
Negative income growth has been found to increase the proneness of a country to civil war outbreak (Collier and Hoeffler 2004), even when accounting for the endogeneity of economic growth in the conflict process (Miguel, Satyanath and Sergenti 2004). It is also found to increase the incidence of violent crime (Fajnzylber, Lederman and Loayza 2002).
Therefore, we would expect negative income shocks to lead to an increased motivation to purchase weapons for the purposes of crime or conflict.