I read an article yesterday that the Sandy Hook shooter "topped off" his AR-15 several times, leaving as many as 15 rounds in discarded magazines. I think this guy played too many video games.
Besides the North Hollywood Shootout and Waco I cannot think of a single "movie style" mass shooting/shootout where hundreds or thousands of rounds were fired.
This whole idea of tracking people who buy ammo is insane...what are you going to do, as a LEO, even if "the creepy guy" in your town is now known to be buying thousands of rounds of ammo and casing local schools? What law is he breaking? How do you "stop' the guy without using man power to follow him around everywhere...and what happens when that "creepy guy" never does a damn thing, mean while your officers are tied up following him around and a shooting happens elsewhere?
The whole idea of somehow regulating "large ammunition purchases" is just going to waste non existant LEO resources.
"....The swords of others will set you your limits".