This is the OP, went out in Jan,Feb in MN (very cold) shooting my pet loads in 30cal with Win760, it seems like I was getting two explosions, KA-BANG not the usual single CRACK. Never experienced this before but I have never shot in this temp 10F. I have been told Win powder need mag primers for cold weather and this might be right. Groups were off slightly.
Went out couple of weeks ago with H4350 with my favorite bullets for this rifle, Hornady and I was impressed. Best 3 shot group ever with this rifle, and it was VERY cold. groups were constant, maybe the Hornady rep was right.