In my experience 120 degrees would not affect a load very much unless you are shooting it while it is that hot.
I have been using shotgun shells in the Imperial Valley and in Southern Arizona for 50 years, and with air temps sometimes approaching 120 degrees the ammo temps coming out of the back of the truck can be up to 185 degrees. Yes, I have a digital thermometer in the truck and I have checked cab and shell temps after the truck has been parked for a while. The shells always work just fine in those temps.
I store my shells in a garage in the sun, and it probably gets to 130 in there on a hot summer day. Shells are fine.
