I am from Ontario and we get the same general whether conditions that time of year as Sask.
For cold conditions while hunting I use a silicone lubricant for my gun. More of a grainy type of lube that does'nt freeze up as much as your best oils.
I have always used a 180 gr bullet, 30.06 for those 70-175 yrd shots. We too have an average 220-250 lbs deer here anda few over 300 lbs. That does the job.