The ones with bullets are going to be best, I'll bet. Powders don't burn right unless they're under pressure, and the temperature is a function of pressure.
I've fired primed brass only into a towel, which resulted in holes and scorching without any ignition of the surrounding area or smoke distinguishable as being from the towel.
There's another experiment I'd like to perform, but I don't have what I need. I'd like to know how exciting cooking off a cartridge base down on a hotplate is. Maybe suspend a piece of plywood a few feet above it. I'm betting that the plywood will be mostly scorched and maybe dented from the slug but nothing major from the slug. My curiousity was aroused in a thread about dropping live cartridges on concrete and, in the unlikely event of a primer strike, how dangerous and spectacular the round going off would be. There's a possibility that the primer alone will discharge the bullet and the majority of the powder will just burn slowly as in the open air.