Bit of talk about this lately and I mentioned it yesterday.
I chrono'd the 77 in the week ... I've chrono'd it over winter on reasonable days outside ... say 10 degrees ... and it's been around the 780 fps with 8.4gr AA and 815 with 7.9. Last week on a bitter cold ( 0 degrees day ) the 8.4 had gone to 815 and the 7.9 to 860 ( yes I know ). It clearly takes the poi higher. I actually brought the 77 inside to warm up and it went back down to 780. Back outside and firing over the chrono you could see it going 780 ... 785 ... 790 ... and up and up. Only seems to happen when it gets proper cold.