What the heck kind of bullets are they shooting? Incendiary rounds?
I have shot all manner of firearms in what can only be described (both practically and officially) as severe drought. Muzzleloaders, handguns, shotguns, high-power rifles, etc. tall dry grass, short dry brush, etc... not even a glimmer of starting a fire.
I have seen the patch from a muzzleloader smoke and smolder on the ground for 30-60 seconds. I have no doubt that under the correct conditions it could easily start a fire.
I have also started fires with flint and steel and know that it is difficult to do. I can not imagine sparks from a modern forearm starting a fire in any weather condition found on earth.