@BillRM,
Bombs are far better killers? Large bombs like Oklahoma city may be better killers but we have already agreed that such bombs are out of the ability of most people to make due to the restrictions on those materials.
Small bombs on the other hand are not better killers of large groups.
The Boston Marathon bombs, there were 2 bombs, killed only 3 people.
With all the bombs that Theodore Kaczynski sent, only 3 people died.
Eric Rudolph only killed 2 people with his bomb during the Atlanta Summer games.
The truck bomb in 1993 in the World Trade Center only killed 6 people.
The fact of the matter is since WW2 there has been maybe 2 bombings, that weren't truck bombs, in the US that killed more than 3 people. In 2015 alone there have been 32 instances where a single attack with guns has killed 4 or more people.
http://shootingtracker.com/wiki/Mass_Shootings_in_2015.