Like Revv said, the heat still gets in. That being said, if you put your glock, or any other "plastic" pistol in there with your important papers, I personally would worry more about the polymer melting than the bullets getting hot enough to do damage.
I personally don't know. I keep mine in two separate safes so I don't have to worry...
"I question things because I am human"
You put your faith in Christianity, I put mine in artillery!