The flash point for gunpowder is something north of 350 degrees and the hottest car on the hottest day is internally 180 degrees or lower. Or something close to that.
Takeaway: ammo is safe to store in hot cars.
I also asked Kart, but meant to ping you too. Thoughts on storing loaded mags in vacuum sealed bags instead of an ammo box with dessicant. This is strictly for in the backpacks in our autos.