The largest wildfire I was ever dispatched to, as a peace officer, was started by 3 individuals shooting lead round balls at a large plowing disc over tall dead grass in August. I was required to take their statements and file an incident report.
The disc was about 50 yards from where they stood, the fire appeared to have started about halfway between them and the disc.
They claimed a ricochet must have caused it, but I doubted a lead ball could cause a spark.
So, yes, I have seen Firsthand where black powder will start a wildfire.
If you don't believe me, simply shoot something flammable (clothe, hay, etc)a few times and see what happens.