There was a fire at an indoor range I used to shoot at years ago.
Maybe fire is too strong a word, it was more like a 'smolder' and the cause was as has been mentioned in many of the other posts, unburned powder, although the initial cause was someone shooting a black powder revolver.
The range was what you might call 'classic' or what you might call 'old and run down' in the basement of an old and run down park pavilion.
A wall with upper and lower stall doors separated the shooters from the actual range. We shot pistol so we used the upper doors, Scouts shot rifle, prone so they would use the lower doors.
The stall doors cut down on the heating cost because only the area on the safe side of the doors was heated. I'm not even sure the range side had a concrete floor, might have been dirt.
In front of the stalls, in the range area there was a plastic tarp about 10 feet wide that ran the width of the range. It made it easier to collect brass that went over the line. We normally never went in front of the shooting line. I should add that there were lights in the safe area and lights at the far end of the range that lit your targets but the rest of the range area was dark.
One night at the end of league shooting one of the members took out a cap and ball six-shooter and asked if he could fire a couple of cylinders of black powder loads. The RO said sure and we all watched him shoot and were really impressed by the amount of smoke produced. As he was reloading, and showing us how it was done, someone noticed the smoke was getting thicker and thicker in the range area. The RO poked his head across firing line into the range area and could dimly see that the plastic tarp had caught on fire and was smoldering with little bits of flare once in a while when the flame hit a pocket of unburnt powder.
Those of us still there opened the lower stall doors, crouched thru and went and stomped out the smolder. No real drama and nobody really blamed the guy with the cap and ball (after all he did ask). Made for just another story to tell. Maybe someday I'll tell about the night a bat got into the range area.