be it fertilizer & fuel oil (OK City), gasoline (37 people in a New Orleans club, 1967), or black powder stripped from firecrackers (Boston Marathon). I support UBCs, but I am under no illusion that such measures will stop mass murderers. NOTE: Banning "assault weapons" (which were never banned) would send a mass murderer to a semi-auto pistol (the VT killer, 32 dead, using standard magazines).
A better approach may be to make settings for mass murders less attractive. Perhaps aggressive self-defense courses should be offered in the same manner as CPR training. The course could deal with ways to tun Toward the problem instead of away. Most folks would not take such a course, but most patrons/shoppers/students do not need such training, but neither do all folks need CPR training. Perhaps a few could at great risk attack the killer and stop him/her and lessen the damage. You may note that in attempts to debunk the idea that an armed "good guy" can stop a mass killer, some in MSM are only to willing to publish accounts of how someone(s) "tackled" and stopped a killer without a gun. So I am not the only one who believes the phenomenon exists.
No advocacy of firearm-only measures is suggested here.
Deal with the problem where it occurs.