I as well, thought that you were supposed to simply drop 1000 UNFIRED bullets on the ground on any map. But what they call "bullets" is actually "cartridge cases". So you just need to shoot 1000 bullets during a single run on any map.

Grab a submachine-gun or an assault rifle (or any gun that is A) automatic, B) that shoots fast (and ideally that you can quickly reload). Buy 1000 bullets of your choice, go to any map and shoot 'em all.

-the Kriss Vector is perfect for the job-

-YOU NEED TO DO THIS DURING A SINGLE RUN-

Also, make sure the durability of your gun is at 100% (so you won't be stopped halfway through because of your gun breaking down).

It doesn't take that much time (I took me about 3 minutes in total), it's just a bit boring.

Have fun litterin' y'all!