It will also surprise you to hear that long guns (i.e. AR-15) are involved only in a small percentage of crimes (including mass shootings) involving firearms. The vast majority of such crimes is committed with handguns, many unlicenced.
https://www.statista.com/statistics/476456/mass-shootings-in...
The definition of "mass shooting" varies, but let's start with this list, and please let me know how it is incorrect in its scope so I can understand your exact point: https://en.wikipedia.org/wiki/List_of_mass_shootings_in_the_...
I would hate for you to be a victim of definitional retreat if you are able to create a well-reasoned argument and back it up with reliable and objective data.
https://reason.com/2012/12/17/are-mass-shootings-becoming-mo...