Two ones in Los Angeles that occurred when I lived there come to mind. I looked up the numbers...:^)
Around 2,000 rounds total (1,300 from the robbers) in the N. Hollywood bank robbery of 1997. The bad guys had full body armour.
The 1974 SLA shootout (remember Patty Hearst) was around 9,000 rounds - probably mostly from the cops in that case.
The SLA shootout apparently was a sort of news TV first...