Compost heaps have been known to heat up to the point of Spontaneous combustion,& Hay when damp,Stacked & then covered also has Spontaneously combusted.I've seen mirrors cook wood planks with tar on them on MythBusters,& I've produced sparks whilst shooting from richocet,I've also ran into 100's of areas in the boonies where, people have set up shooting areas with tons of glass shards blown about,there is always the chance of Magnifacation on clear windless days,of which there are many in the desert!