During my lifetime, I've been on many shooting ranges, both indoor and outdoor, and either seen fires, or the results of them. Little thought is given to fires other than the usual safety of warming fires, where permitted, and limiting the amount of blackpowder on the firing line at any one time. But they occur, much to the surprise of the participants.
Several years back I was firing one pleasant fall morning at our public outdoor range. I was on the left hand side, for handguns, when I noticed a familiar bright red streak go down range, a tracer. There followed very quickly a second streak, which hit the berm downrange and bounced up into the air about fifty feet. Within a few minutes, thick gray smoke began rolling up from the dry leaves and burst into flame. The range officer immediately cleared the firing line, and when clear, a class of security guards in training went downrange and extinguished the fire. Clearly military tracer and incendiery ammunition should not be used on public shooting ranges. Most states prohibit this type of ammunition on state park ranges, and it is banned from hunting use in all states that I know of.
On several occassions I have witnessed fires spring up near the firing line, ignited from a dropped cigarette or sparks from muzzleloaders' patches, maybe even from muzzle flashes as well. Several ranges I have seen have an asphalt topping along the firing line. This, often covered, slopes away toward downrange for drainage. Unburned powder, from both rifles and handguns, settles on this apron, and is washed by rainwater into a drainage ditch. Wetting this powder does not neutralize it, it remains flammable when dried out. A spark, a dropped cigarette or cigar, is all it takes to flare up a brief fire.
Yet another bizare tale was told me by the owner of an indoor firing range. There was a large patch of spalled concrete a few feet down range, and I asked him about it. He told me that unburned powder had collected there, sort of swept into a pile by the ventilation system. A shooter was firing when apparently a spark from his muzzle blast ignited the pile and it exploded. Powder, unconfined, does not explode, but burns very fast and very hot. What had happened was that the burning powder turned the moisture in the concrete into steam and the expanding steam popped the surface layer of concreete loose. To verify that his occurs, look at the concrete floor in a welding shop where little craters are visible in the concrete floor where a torch has been held against the floor momentarily, either inadvertently or purposefully, to get that POP! sound.
Obviously, prohibit tracer and incendiary ammunition from public shooting ranges. Also, prohibit smoking on the firing line, a rule that is getting more easily enforced nowadays with anti-smoking campaigns throughout the country. And, clean up often, either with a high pressure wash-down, or vacuuming. Be aware that that vacuum bag is full of very flammable powder after awhile, and dispose of it safely. Usually, local fire marshals are willing to help with advice on clean-up. And, the NRA offers guidance in operating shooting ranges.
An ounce of prevention..........