If it makes the gun to hot to handle, it's too hot to shoot. That is my general rule of thumb.
Also, like M2 I had heat stroke last year and was hospitalized. It is not fun at all, and it does sneak up on you VERY suddenly. And the most bizarre thing about my experience was I was in air conditioning most of the day when it happened. I had just eating a large burger 45 minutes before it happened, and the body uses a fair amount of water to digest protein. So that is probably what pushed my body over the limit. The Dr at the hospital stated that I had likely been chronically dehydrated for weeks or months before.
Since that time, I tend to no enjoy going out when the heat index is above 95. But almost every day of this summer as been 100+, which is highly unusual for my area, even in the south east (we get days over a 100, but not for frickin months on end).
If you are going out, carry a gallon or three of water along with you. And I know a cold beer is great after a day of work or shooting in the heat, but if you are going to do that drink plenty of water before and after. Alcohol is a diuretic.