Firing ranges on military bases aren't anything like those WW I battlefields in France. But still the number of bullets, fragments, and yes, unexploded ordnance, can be quite astounding.
Years ago contractors actually paid good money to get to go out to the berms (piled up dirt walls) behind rifle and pistol ranges, to dig out and screen the tons of pistol and rifle bullets. Copper and lead: both good scrap sellers.
The artillery ranges are different: they can be quite dangerous because of the duds on the surface and shallowly buried. Nobody wants to dig up duds :-)
Good point, someone above: are the seeds going to sprout plants that become a pest or menace in themselves?