I hadn't thought of North Hollywood. I read that about 1,100 rounds were fired there. (I believe they had three times as much ammo as they actually fired.) Ironically, there were no other fatalities in that incident, beside the perpetrators.
There were two of them, and they had an automatic weapon. And yet, the total amount fired still wasn't all that much in comparison to what people regularly purchase for ordinary use. And when people are as well-organized as these guys were, they can easily stockpile small purchases until they have all they need.