I stumbled across this video from SAAMI, intended for firefighters, that runs a series of experiments on a variety of loose ammo. They hit it with hammers, drop it from a crane, run it over with a bulldozer, shoot it with another bullet, and at about 13 minutes into the video they finally start setting it ablaze.
It should be noted that they're using some pretty serious calibers in some of these tests. And in the first test, the casings are anchored against the wall of the test box which undoubtedly helped the bullets' velocity.
It seems about every week someone is posting a question about loose ammo going off in a fire, or igniting some other way. Finally, some scientific evidence!
If you don't want to watch the video, suffice it to say that loose ammo is pretty safe in a fire, doesn't have much velocity, and is difficult to ignite in many of the other tests.