Jim, I have been doing this with rifle, shotgun and pistol ammunition with no ill effect to date. I do make it a practice to rotate out vehicle stored ammunition (shot at the range or when plinking outside) with fresh ammunition periodically. Components and powders are much more temperature stable today than in years past.

Jim, Components used in ammunition manufacturing are vastly improved to what was used even 25 years ago. If you rotate your ammo out to use and replace every year or so no problem.

I can assure that ammo that we shot in the military was sometimes exposed to very intense heat (Metal ammo bunker) and we never had any issues