I toured the Nosler plant twice, back when they had public tours, and I was in the ballistics lab both times. It was a great facility and some really nice people.
It's pure speculation on my part, but since the story says it started in the ballistics lab, my guess is it was started by a flash fire of unburned powder in front of the shooting bench at the entrance to the tunnel. This is pretty common on indoor ranges when they don't wet mop the floors as often as they should. There can be quite an accumulation of powder on the floor directly in front of the benches. On outdoor ranges the wind and weather pretty much take care of it, but on indoor ranges it accumulates, especially if there are cracks in the concrete, such as stress seams, etc.
And as I said, this is pure speculation on my part.
Fred
