It depends on WHERE he was shooting.
If he was at a formal range, probably not much chance of starting a fire.
But, if he was shooting at one of the "stop and shoot" type "ranges," which aren't formal ranges at all but just secluded areas where people have gathered to shoot, there could be rocks, old cars, appliances, etc., all over the place.
I was out shooting at such a range in Pennsylvania with friends some years ago.
There was an old car there that we were shooting at.
I pulled out my SKS, and started popping away. This was in the day of Russian surplus steel-core ammo, so you can imagine the sparks that were being kicked up.
It's also not inconceivable that the person was shooting tracers, too. They're out there, and pretty cheap, and are fully capable of starting a fire.