According to the Internet Movie Firearms Database’s crowd-sourced data collection, gun violence in PG-13 films has more than tripled since 1985 (via The Economist); in recent years, that number has even exceeded the violence of R-rated films.
The increase in onscreen gun violence can certainly be attributed to a larger genre shift; in the 1930s, action movies made up only 4% of the highest-grossing films; now that number is around 34%.
However, considering the visibility cinema gives to firearms and its subsequent effects on real-world markets, a conversation certainly needs to be had as to the potential effects of having such levels of visibility in PG-13 films.
And it looks as if the numbers may actually back up this view.