We should be able to agree here, at least, that commonly cited counts of Parkland-style mass shootings are overstated, and that the prevalence of tactical rifles in those kinds of mass shootings are not.
Of the 28 mass shootings of 10 or more victims in the last 50 years, 14 of them list the use of a semi automatic rifle or carbine, so while I agree that changes the numbers a bit, I don't think 50% is as conclusive as you're letting on.
This is from wikipedia, so take it with a grain of salt if you'd like.
The percentage goes up if you select more recent shootings (say, since 2000, inconveniently leaving Columbine behind), which makes sense, because semi-automatic rifles have become more prevalent since then. And 20 years is still a plenty long time ago.
