I walked up to the local ERT while at the range (decked out head to toe in tac gear, AR15 strapped to my back, Glock in the holster. Asked one of the guys what they did with the brass. He said they just leave it. Sat on a bench for ten minutes while they loaded up, put my gear down and filled a couple five gallon pails with .223 and 9mm. Got 50 once fired Federal GMM cases from the department snipers too!
The cop seemed a bit miffed at why I needed so much brass, "what are you ever gonna do with two pails full?" "Load it and shoot it" I said, as if the tac vest with 22 AR mags on me wasn't a dead giveaway that I did spend a bit of time at a gunrange. The police officer was kind enough to let me know the next time they were out training as well, managed another pail or so of once fired brass.
If you see ERT, police training at your range, walk over, introduce yourself and ask for the leftover brass, I have never seen a cop pick up a single piece of brass or a shotgun hull around here, even if its a loaded round that gets dropped. Great source of once fired brass, and we paid for, just go ask for it back!