Are you shooting while there or taking so much ammo anticipating a small scale war? Temporary temp extremes should not have an advese affect on your ammo. I would not store it in the trunk idefinitely though. LOng term, bad.
"Given adequate penetration, a larger diameter bullet will have an edge in wounding effectiveness. It will damage a blood vessel the smaller projectile barely misses. The larger permanent cavity may lead to faster blood loss. Although such an edge clearly exists, its significance cannot be quantified".