The San Jose shooter had 22000 rounds of ammunition in his house. Perfectly normal and no need to do anything.
https://www.bbc.co.uk/news/world-us-canada-57293174
I've always found it strange, probably because I'm not a crazy homicidal nutcase, how often with these mass shootings they've gone off quarter cocked so to speak despite having a plan and equipped for a much bigger act - not that I want something worse to happen just find it odd. Maybe they get paranoid someone is on to them and think they have to act before they get stopped or maybe something triggers them and they can't control the impulse or whatever.
Obviously in some cases things won't go down as they planned, etc. whether the law enforcement response stops them or mentally they weren't prepared for the reality of it once they'd started, etc. etc. but been quite a few cases where they've assembled explosives, got themselves a bunch of rifles and so on and lots of ammo but then carried out an act with "just" a couple of handguns and a dozen rounds or so.