Seems like an awful lot of guns for the time he could have expected to have, then.
Like a lot of amateur preppers stockpiling guns never really realizing that they can only use one effectively at a time and if they have to move quickly, they can only carry one, or two with them.
To do the deed meant that those reconstituted guns with the whatchamacallit added, to make them fully automatic, would cause them to over heat, thus, more guns would be needed, and also reportedly two of them were found jammed.
A concert venue of that size as the target was definitely going to require a lot of guns for achieving maximum death.
The more guns prepared and the faster he fired them as fully automatic, then the more ammo was used which increased chances of downing more victims in the shortest amount of time, before either being caught, or able to escape.