Just a couple of years ago there was a post on a predator hunting board where a guy had a box of pistol ammo. sitting on the dash of his vehicle while he was out hunting. When he got back to the truck, a couple of the shells had gone off, but for the most part, the box that the shells were in sustained the only damage.
It's obvious that putting any ammo. on the dash of a vehicle is asking for trouble because of the obvious heat factor that can occur. A guy has to be careful with ammo., and heat.