Bullets go faster in thinner air. Chamber pressure goes up in thinner air. I really don't think its the temperature at all. I mean if you leave the ammo in the sun until its 150 degrees then that is an issue. Or vice versa leaving your ammo in a freezing car overnight, you will start to see velocity reductions. But you could bring the cold ammo in the chamber for a few extra seconds on a warm barrel and bring it up to summer temps even then. Cold ammo is much worse than hot ammo. Hot ammo shoots good. But that little bit of time the ammo is exposed to external temps is not what most people are seeing that moves them out of the node, or takes them over the top of max pressure.