I was at the range a trifle early Saturday morning, an indoor range. The owner was downrange, scraping tape off the floor. I asked him about this, and he told me instructors often marked off firning points for their various classes with this tape. They are asked to remove it when finished, but many fail to do this.
Pointing out an area of spalled concrete about 30" x 18", he told me tape had been left there and unburned powder had collected in that area. Somehow, a stray spark had ignited that accumulation, the resulting explosion had blown that hole in the concrete sending fragments into the acoustic tile above, which showed damage as well.
This didn't quite ring true to me, yet I don't doubt the integrity of the man who told me the story. We all know unconfined smokeless powder doesn't explode, it burns fiercely, but it does not explode.
What I think happened is that the powder did ignite, and that heat of the burning powder caused moisture in the concrete to turn to steam and explode, much the way a damp brick or stone will explode when exposed to fire. Sreel fabrication shops often have small holes in the concrete floor from holding torches close to the surface, either accidental or intentional, popping small holes into the concrete.
Some time back, at an outdoor range, a fire suddenly erupted the length of the firing line, burning bright and fierce for a few seconds. The firing line was asphalt, sloped downrange for drainage. Rain had washed unburned powder into a shallow ditch in front of the firing line. A stray spark, from maybe a muzzle loader, had ignited the accumulation of unburned powder. It burned itself out in a few seconds. leaving some dry grass and leaves still smoldering. But though rain had dampened that powder, it was still just as flammable when dried out.
These are isolated and rare instances, but the fact remains, they did happen. A word to the wise.