Except by that exact source, mass shootings with rifles are under reported and the deadliest mass shootings were done with semi automatic rifles.
“Since 1982, there has been a known total 65 mass shootings involving rifles, mostly semi-automatics. This figure is underreported though, as it excludes the multiple semi-automatic (and fully automatic) rifles used in the 2017 Las Vegas Strip massacre – the worst mass shooting in U.S. history, killing 58 and wounding 546. In fact, semi-automatic rifles were featured in four of the five deadliest mass shootings, being used in the Orlando nightclub massacre, Sandy Hook Elementary massacre and Texas First Baptist Church massacre.”
I can’t say for sure whether or not this particular study used proper testing, but as a whole introversion and extroversion is not pseudoscientific.
Jung wasn’t a good scientist, but he did a lot of studies and came up with a lot of theories, some of which happened to be at least partially correct. Also, you seem to be getting something mixed up because Jung defined introversion as an “attitude-type characterised by orientation in life through subjective psychic contents”, and extraversion as “an attitude-type characterised by concentration of interest on the external object”, whereas the more common energy focused definition is not from Jung at all - at least, as far as I am aware.
The big five personality traits, namely openness, conscientiousness, extraversion, agreeableness, and neuroticism have been shown to be consistent, even cross culturally.
There are limitations to that: like how it’s an empirical observation, that other personality traits exist that aren’t factored into those five, or that it’s possible there are a larger number of smaller subfactors that make up those five traits, but ultimately they are scientific.