The first is a correlation study regarding the introduction of semi-automatic handguns vs revolvers (included would be an examination of trigger sensitivity between the two - for those that don't know, typically, the trigger pull on a semi is lighter than on a revolver).  This study should also include the influence of having the availability of more cartridges when it comes to semi's and revolvers, which would include the increased ease of reloading in a semi to that of a revolver.

(Here is a funny thing about revolvers and semis - when I started out, our dept was in transition from revolvers to semi-autos and many of the remaining revolver shooters were the most accurate with a handgun.  I think the attributes that influenced that was not only was a more steady hand needed for a good revolver shot, but those shooters also knew in their minds that they had just six shots and they needed to make sure each one was as accurate as possible.)

The next would involve the emergence of "shoot 'em up" video games over the years.  While I do not believe video games are a causal link to deviance, what I do wonder about is if all of these video games and their endless supply of shooting capabilities are sort of a gateway to what may later lead to more of a prevalence of being willing to pull a trigger more often and quicker, especially if the expected result once seen in a video game is not immediately seen in real life.