What is in question is whether this kid could have killed more people with a different weapon. Arguing that weapon type doesn't matter isn't ignorance; it's you being disingenuous and dishonest.
The argument that the .223/5.56 SBR kills better than a pistol caliber has been addressed in numerous FBI reports on mass shootings (in addition to their own, internal studies for arming). In directly comparing many 9mm v. 5.56mm mass shootings at close range, we see more of a 1:1 ratio of kill-wounded by 9mm than 5.56mm, totally against common assumption. In fact, most 5.56mm engagements have resulted in 1:4 or worse!
That's because all of these engagements are close range, where the pistol rounds don't lose virtually any energy, all while having 3-4x the cross section, plus the common civilian loads that aren't idea for hunting (at range) that most rifle cartridges utilize.
Terminal energy does not always equal greater kills. It almost always results in greater wounding, when there is not a kill, but not necessarily kills.
Furthermore, the popular assumption that a semi-auto 5.56mm shoots faster than a 9mm has to end. Even the Florida shooter only used 10 cartridge magazines, and a typical 9mm carries more, or in a compact form, usually just as many. Although he did get closer to a 1:1 ratio, which is very rare (but he seemed well trained).
This is what bothers most of us ... because they show an AR-15 icon, even when a Glock is used -- because both are 'assault weapons.'