I've carried an MP5 with all its plastic parts in the trunk of a squad car in the heat of August in Texas for 12-15 hours a day. No problems.
Same with Glocks.
There's an AR sitting straight up in my enclosed squad car up to 10-18 hours a day. It's not only battling the heat but the visible sunlight. Nothing. Sometimes it will sear your fingerprints a little when you grab it.
Hard armor vest with 120 extra 5.56 rounds. Nope. They all still go bang.
Its way hotter inside a Humvee all over the Middle East and there are Glocks, Berettas, M4's and a bunch of other partially plastic guns plodding along without a problem.
I have killed off some .22 ammo stored loose in an ammo can in the trunk of my car and garage here in Texas. It seems the wax lube can melt and deactivate the primer. Out of a couple thousand rounds, after about 15 years in a garage, I found them. Some were a little stuck together. Maybe one dud in every 100 rounds.