With decreasing temp, areas I look at is powder and powder charge. Is the powder temp sensitive? If so, by how much. sometimes a load in heat of summer is 50 to 100fps slower in the fall. Accuracy, POI and drops would all change.
I use Hodgdon Extreme powders as much as possible and that seems to help. Recent use of Re25 also show nice stability.
One thing we can't control is the primer. Brisance of primers play a large part in the overall pressure of each load. I used to think that velocity and pressure were linear. Recent pressure tests show otherwise. You can have much lower chamber pressures at very similar velocities. You got to test to know for sure.
Also, some loads are run at 'normal' pressures. Once the powder or primer 'cools off', they may not burn optimally. Sort of like saying that you are falling off the sweet spot of the powder. Like using 1/2 grain less powder.
In general, my hunting loads work at elevated pressures and I don't seem to find as big a problem.
The next area to consider are the optics and what you see. Summer time will always have mirage and usually way brighter. We know that light intensity and location can cause our apparent POI to shift. My zero settings on my 180yd range is always a bit left then my 100yd range. I blame that on the lighting. Both ranges are less then 100yds apart.
Calmer fall air and lower temps change what we see and what the bullet experiences. I would have expected a wash but some rifles react more then others. Some bullets are affected more as well. You just have to test.
The final is the rifle material itself. Some barrels change their harmonics and POI when very cold. My last hunting rifle in '06 was wonderfully consistent throughout the shooting year.
Took it to a fun shoot and it was brutally cold. The POI changed over 1min at 100yds. Drove me insane. I did the stupid thing of not sighting in when I arrived. After a few shots and chasing the scope, the rifle went back to its orig POI. I never knew that barrel changed that much in the cold because I didn't shot in those temps.
Also, stocks do contract in the cold. That affects bedding pressure and how it reacts in recoil. These are all very small changes but that can all lead to the discrepancies listed.
Yes, 1/2MOA can lead to a miss at LR. The worse part is that you don't know whats going on so can't adjust. After that first shot goes off and if its a miss or a wounding shot, most don't stay calm and relaxed. Usually many more shots are launched in quick succession with varying results. At that point, panic and despair/frustration sets in.
If the game is lost or wounded for an extended period of time, most hunters will feel really bad and the loss of confidence in gear and shooter can last a very long time.
The only way to reduce this is to shoot under the conditions you intend to hunt in. Not always easy. At the least, make some time when you arrive to do a quick test.
I like to confirm my 100yd or drop chart zero. Then I will range, dial up and put a few at different distances. If all is well, I am good to go.
Hunts usually end positively when you KNOW where your bullet is going to arrive.
Jerry
