The pistol ranges where I shoot have covered concrete pads at the firing points and gravel out to the target. Most people shoot smokeless. Apparently, a significant amount of unburned smokeless powder accumulates in the gravel in the few inches just off the edge of the concrete--probably either falls there or gets swept there off of the concrete.
I have twice ignited this accumulated powder with stray sparks from black powder cartridges--no grease wads, cards, or paper in the cartridges. The first time caused a little bit of pucker as it lit in the middle of the firing point and starting burning both directions toward the wooden walls. It was a pretty small flame, but hard to stomp/kick out, because that just seemed to kick in more unburned powder. Fortunately, being smokeless (and probably somewhat weathered), the flame moved pretty slowly.
The second time it happened we just immediately went to the left and right edges of the pad, kicked up fire breaks to keep it away from the wood, and let it burn itself out rather than trying to extinguish the flames.
With the grass and weeds at the edges of the range, a windy day during summer could have complicated things quite a bit.