I only see this from mechanical and physics stand point.
As Temps drop, many lubricants thicken up with some becoming sticky = Drag & this includes lubes on the projectile.
Air density increases ... So, air getting pushed out ahead of projectile absorbs a tad bit of energy in the cold. Of course said projectile flying threw denser air will have a lower BC. Air being displaced by the hammer moving forward as well any vacuum on the backside also absorb a bit more energy.
Barrel bore in the cold will be ever so slightly smaller/tighter, as could be the bore hammer slides within.
IMO, if one tried to measure each potential effect, you likely could not, but it is still present & mentioned reasons ALL combined together you may very well find slight damped power output in the cold over warmer conditions.
Some AG's pending design, metals/materials made from, lubes used or not used could have near zero effect in the cold, while other may really show measurable deficit.
JMO ...
